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>

tests/unit/test_navigation_template.py

@@ -2,7 +2,7 @@ import unittest
 from html.parser import HTMLParser
 from pathlib import Path
 
-from jinja2 import Environment, FileSystemLoader, select_autoescape
+from jinja2 import Environment, FileSystemLoader
 
 
 class AnchorCollector(HTMLParser):
@@ -20,10 +20,12 @@ class TestNavigationTemplate(unittest.TestCase):
         template_dir = Path(__file__).resolve().parents[2] / "app" / "templates"
         environment = Environment(
             loader=FileSystemLoader(template_dir),
-            autoescape=select_autoescape(),
+            autoescape=True,
         )
-        environment.globals["url_for"] = lambda _endpoint, filename: (
-            f"/static/{filename}"
+        environment.globals["url_for"] = lambda endpoint, **kwargs: (
+            f"/static/{kwargs['filename']}"
+            if endpoint == "static"
+            else f"/{kwargs['lang']}/"
         )
         environment.globals["asset_src"] = lambda asset: (
             (asset or {}).get("external_url")
@@ -36,6 +38,9 @@ class TestNavigationTemplate(unittest.TestCase):
 
         rendered = environment.get_template("moduls/navigation.html.j2").render(
             menu_type="header",
+            lang="en",
+            languages={"en": "English", "de": "Deutsch"},
+            t=lambda source: source,
             platform={
                 "titel": "Portfolio",
                 "logo": {"cache": "logo.png"},
@@ -70,8 +75,15 @@ class TestNavigationTemplate(unittest.TestCase):
             and "dropdown-toggle" in anchor.get("class", "")
         ]
 
-        self.assertEqual(len(dropdown_toggles), 1)
-        self.assertEqual(dropdown_toggles[0].get("data-bs-toggle"), "dropdown")
+        self.assertEqual(len(dropdown_toggles), 2)
+        for toggle in dropdown_toggles:
+            self.assertEqual(toggle.get("data-bs-toggle"), "dropdown")
+
+        language_links = [
+            anchor for anchor in parser.anchors if anchor.get("hreflang") == "de"
+        ]
+        self.assertEqual(len(language_links), 1)
+        self.assertEqual(language_links[0]["href"], "/de/")
 
 
 if __name__ == "__main__":