feat(i18n): serve every page in 30 languages

The interface ships translated; page content stays English until a
LibreTranslate instance fills app/i18n/content/ through make i18n. A string
without a catalogue entry falls back to its English source, so a half-filled
catalogue degrades instead of breaking.

Translation runs after ConfigurationResolver.resolve_links(), on a copy.
resolve_links matches by the `name` field, so translating it beforehand
would break every `link:` reference in the configuration.

negotiate() normalises to the primary subtag itself. Werkzeug's best_match
returns an exact match before it considers a primary-tag fallback, so the
Chrome default `de-DE,en;q=0.8` resolves to English there. "/" carries
Vary: Accept-Language, without which a shared cache pins the first
visitor's language for everyone.

The route rule lists the known codes as a converter argument. A bare
"/<lang>/" answers /robots.txt and /favicon.ico with a permanently
cacheable 308 to their trailing-slash form.

Templates gain lang, dir, the RTL stylesheet, a canonical URL and 30
hreflang alternates. Those are the first external URLs in this app:
ProxyFix takes the scheme from X-Forwarded-Proto so they do not claim
http:// behind a TLS-terminating proxy, X-Forwarded-Host stays untrusted
because nginx passes a client-supplied one through, and TRUSTED_HOSTS lets
Flask reject a forged Host outright.

Flask only autoescapes .html/.htm/.xml/.xhtml/.svg, so every *.html.j2
template interpolated configuration raw. Enabling it changes two lines of
the shipped page, both an apostrophe.

read_catalog degrades an unreadable catalogue to English rather than
serving a 500, and drops non-string entries that would otherwise render as
"42". i18n_sync writes atomically, never overwrites an existing entry,
refuses to touch a catalogue it could not parse, and leaves the file alone
when a run translated nothing.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/i18n/ui/ar.yaml

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+Alternatives: بدائل
+Close: إغلاق
+Copy: نسخ
+Identifier copied to clipboard!: تم نسخ المعرّف إلى الحافظة!
+Imprint: معلومات قانونية
+Information: معلومات
+Language: اللغة
+Open: فتح
+Open Link: فتح الرابط
+Options: خيارات
+Warning: تحذير

app/i18n/ui/bn.yaml

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+Alternatives: অন্যান্য বিকল্প
+Close: বন্ধ করুন
+Copy: অনুলিপি
+Identifier copied to clipboard!: শনাক্তকারী ক্লিপবোর্ডে অনুলিপি করা হয়েছে!
+Imprint: আইনি তথ্য
+Information: তথ্য
+Language: ভাষা
+Open: খুলুন
+Open Link: লিঙ্ক খুলুন
+Options: বিকল্প
+Warning: সতর্কতা

app/i18n/ui/cs.yaml

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+Alternatives: Alternativy
+Close: Zavřít
+Copy: Kopírovat
+Identifier copied to clipboard!: Identifikátor zkopírován do schránky!
+Imprint: Tiráž
+Information: Informace
+Language: Jazyk
+Open: Otevřít
+Open Link: Otevřít odkaz
+Options: Možnosti
+Warning: Upozornění

app/i18n/ui/da.yaml

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+Alternatives: Alternativer
+Close: Luk
+Copy: Kopiér
+Identifier copied to clipboard!: Identifikator kopieret til udklipsholder!
+Imprint: Juridisk information
+Information: Information
+Language: Sprog
+Open: Åbn
+Open Link: Åbn link
+Options: Valgmuligheder
+Warning: Advarsel

app/i18n/ui/de.yaml

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+Alternatives: Alternativen
+Close: Schließen
+Copy: Kopieren
+Identifier copied to clipboard!: Kennung in die Zwischenablage kopiert!
+Imprint: Impressum
+Information: Information
+Language: Sprache
+Open: Öffnen
+Open Link: Link öffnen
+Options: Optionen
+Warning: Warnung

app/i18n/ui/el.yaml

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+Alternatives: Εναλλακτικές
+Close: Κλείσιμο
+Copy: Αντιγραφή
+Identifier copied to clipboard!: Το αναγνωριστικό αντιγράφηκε στο πρόχειρο!
+Imprint: Νομικές πληροφορίες
+Information: Πληροφορίες
+Language: Γλώσσα
+Open: Άνοιγμα
+Open Link: Άνοιγμα συνδέσμου
+Options: Επιλογές
+Warning: Προειδοποίηση

app/i18n/ui/es.yaml

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+Alternatives: Alternativas
+Close: Cerrar
+Copy: Copiar
+Identifier copied to clipboard!: ¡Identificador copiado al portapapeles!
+Imprint: Aviso legal
+Information: Información
+Language: Idioma
+Open: Abrir
+Open Link: Abrir enlace
+Options: Opciones
+Warning: Advertencia

app/i18n/ui/fa.yaml

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+Alternatives: جایگزین‌ها
+Close: بستن
+Copy: کپی
+Identifier copied to clipboard!: شناسه در کلیپ‌بورد کپی شد!
+Imprint: اطلاعات حقوقی
+Information: اطلاعات
+Language: زبان
+Open: باز کردن
+Open Link: باز کردن پیوند
+Options: گزینه‌ها
+Warning: هشدار

app/i18n/ui/fi.yaml

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+Alternatives: Vaihtoehdot
+Close: Sulje
+Copy: Kopioi
+Identifier copied to clipboard!: Tunniste kopioitu leikepöydälle!
+Imprint: Oikeudelliset tiedot
+Information: Tiedot
+Language: Kieli
+Open: Avaa
+Open Link: Avaa linkki
+Options: Valinnat
+Warning: Varoitus

app/i18n/ui/fr.yaml

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+Alternatives: Alternatives
+Close: Fermer
+Copy: Copier
+Identifier copied to clipboard!: Identifiant copié dans le presse-papiers !
+Imprint: Mentions légales
+Information: Informations
+Language: Langue
+Open: Ouvrir
+Open Link: Ouvrir le lien
+Options: Options
+Warning: Avertissement

app/i18n/ui/he.yaml

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+Alternatives: חלופות
+Close: סגור
+Copy: העתק
+Identifier copied to clipboard!: המזהה הועתק ללוח!
+Imprint: מידע משפטי
+Information: מידע
+Language: שפה
+Open: פתח
+Open Link: פתח קישור
+Options: אפשרויות
+Warning: אזהרה

app/i18n/ui/hi.yaml

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+Alternatives: अन्य विकल्प
+Close: बंद करें
+Copy: कॉपी करें
+Identifier copied to clipboard!: पहचानकर्ता क्लिपबोर्ड पर कॉपी हो गया!
+Imprint: कानूनी सूचना
+Information: जानकारी
+Language: भाषा
+Open: खोलें
+Open Link: लिंक खोलें
+Options: विकल्प
+Warning: चेतावनी

app/i18n/ui/hu.yaml

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+Alternatives: Alternatívák
+Close: Bezárás
+Copy: Másolás
+Identifier copied to clipboard!: Az azonosító a vágólapra másolva!
+Imprint: Impresszum
+Information: Információ
+Language: Nyelv
+Open: Megnyitás
+Open Link: Hivatkozás megnyitása
+Options: Lehetőségek
+Warning: Figyelmeztetés

app/i18n/ui/id.yaml

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+Alternatives: Alternatif
+Close: Tutup
+Copy: Salin
+Identifier copied to clipboard!: Pengenal disalin ke papan klip!
+Imprint: Informasi hukum
+Information: Informasi
+Language: Bahasa
+Open: Buka
+Open Link: Buka tautan
+Options: Opsi
+Warning: Peringatan

app/i18n/ui/it.yaml

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+Alternatives: Alternative
+Close: Chiudi
+Copy: Copia
+Identifier copied to clipboard!: Identificatore copiato negli appunti!
+Imprint: Note legali
+Information: Informazioni
+Language: Lingua
+Open: Apri
+Open Link: Apri collegamento
+Options: Opzioni
+Warning: Avviso

app/i18n/ui/ja.yaml

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+Alternatives: 代替
+Close: 閉じる
+Copy: コピー
+Identifier copied to clipboard!: 識別子をクリップボードにコピーしました！
+Imprint: 法的情報
+Information: 情報
+Language: 言語
+Open: 開く
+Open Link: リンクを開く
+Options: オプション
+Warning: 警告

app/i18n/ui/ko.yaml

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+Alternatives: 대안
+Close: 닫기
+Copy: 복사
+Identifier copied to clipboard!: 식별자를 클립보드에 복사했습니다!
+Imprint: 법적 고지
+Information: 정보
+Language: 언어
+Open: 열기
+Open Link: 링크 열기
+Options: 옵션
+Warning: 경고

app/i18n/ui/nl.yaml

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+Alternatives: Alternatieven
+Close: Sluiten
+Copy: Kopiëren
+Identifier copied to clipboard!: Identificatie gekopieerd naar klembord!
+Imprint: Colofon
+Information: Informatie
+Language: Taal
+Open: Openen
+Open Link: Link openen
+Options: Opties
+Warning: Waarschuwing

app/i18n/ui/pl.yaml

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+Alternatives: Alternatywy
+Close: Zamknij
+Copy: Kopiuj
+Identifier copied to clipboard!: Identyfikator skopiowany do schowka!
+Imprint: Nota prawna
+Information: Informacja
+Language: Język
+Open: Otwórz
+Open Link: Otwórz link
+Options: Opcje
+Warning: Ostrzeżenie

app/i18n/ui/pt.yaml

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+Alternatives: Alternativas
+Close: Fechar
+Copy: Copiar
+Identifier copied to clipboard!: Identificador copiado para a área de transferência!
+Imprint: Aviso legal
+Information: Informação
+Language: Idioma
+Open: Abrir
+Open Link: Abrir link
+Options: Opções
+Warning: Aviso

app/i18n/ui/ro.yaml

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+Alternatives: Alternative
+Close: Închide
+Copy: Copiază
+Identifier copied to clipboard!: Identificator copiat în clipboard!
+Imprint: Notă legală
+Information: Informații
+Language: Limbă
+Open: Deschide
+Open Link: Deschide linkul
+Options: Opțiuni
+Warning: Avertisment

app/i18n/ui/ru.yaml

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+Alternatives: Альтернативы
+Close: Закрыть
+Copy: Копировать
+Identifier copied to clipboard!: Идентификатор скопирован в буфер обмена!
+Imprint: Выходные данные
+Information: Информация
+Language: Язык
+Open: Открыть
+Open Link: Открыть ссылку
+Options: Варианты
+Warning: Предупреждение

app/i18n/ui/sv.yaml

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+Alternatives: Alternativ
+Close: Stäng
+Copy: Kopiera
+Identifier copied to clipboard!: Identifierare kopierad till urklipp!
+Imprint: Juridisk information
+Information: Information
+Language: Språk
+Open: Öppna
+Open Link: Öppna länk
+Options: Val
+Warning: Varning

app/i18n/ui/th.yaml

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+Alternatives: ทางเลือกอื่น
+Close: ปิด
+Copy: คัดลอก
+Identifier copied to clipboard!: คัดลอกตัวระบุไปยังคลิปบอร์ดแล้ว!
+Imprint: ข้อมูลทางกฎหมาย
+Information: ข้อมูล
+Language: ภาษา
+Open: เปิด
+Open Link: เปิดลิงก์
+Options: ตัวเลือก
+Warning: คำเตือน

app/i18n/ui/tr.yaml

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+Alternatives: Alternatifler
+Close: Kapat
+Copy: Kopyala
+Identifier copied to clipboard!: Tanımlayıcı panoya kopyalandı!
+Imprint: Künye
+Information: Bilgi
+Language: Dil
+Open: Aç
+Open Link: Bağlantıyı aç
+Options: Seçenekler
+Warning: Uyarı

app/i18n/ui/uk.yaml

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+Alternatives: Альтернативи
+Close: Закрити
+Copy: Копіювати
+Identifier copied to clipboard!: Ідентифікатор скопійовано в буфер обміну!
+Imprint: Вихідні дані
+Information: Інформація
+Language: Мова
+Open: Відкрити
+Open Link: Відкрити посилання
+Options: Варіанти
+Warning: Попередження

app/i18n/ui/ur.yaml

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+Alternatives: متبادل
+Close: بند کریں
+Copy: نقل کریں
+Identifier copied to clipboard!: شناخت کنندہ کلپ بورڈ پر نقل ہو گیا!
+Imprint: قانونی معلومات
+Information: معلومات
+Language: زبان
+Open: کھولیں
+Open Link: لنک کھولیں
+Options: اختیارات
+Warning: انتباہ

app/i18n/ui/vi.yaml

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+Alternatives: Lựa chọn thay thế
+Close: Đóng
+Copy: Sao chép
+Identifier copied to clipboard!: Đã sao chép mã định danh vào bộ nhớ tạm!
+Imprint: Thông tin pháp lý
+Information: Thông tin
+Language: Ngôn ngữ
+Open: Mở
+Open Link: Mở liên kết
+Options: Tùy chọn
+Warning: Cảnh báo

app/i18n/ui/zh.yaml

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+Alternatives: 替代方案
+Close: 关闭
+Copy: 复制
+Identifier copied to clipboard!: 标识符已复制到剪贴板！
+Imprint: 法律声明
+Information: 信息
+Language: 语言
+Open: 打开
+Open Link: 打开链接
+Options: 选项
+Warning: 警告