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/fi.yaml

@@ -0,0 +1,11 @@
+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