feat(i18n): add internationalization framework with 10 languages

- Create i18n utility module (app/utils/i18n.py) with translation loading,
  browser language detection, AI fallback, and l10n helpers
- Add JSON translation files for EN, DE, FR, ES, IT, PT, NL, PL, ZH, RU
- Add preferred_language column to UserProfile model with migration
- Register _() translation function as Jinja2 global
- Update base.html with translated navigation, footer, cookie notice
- Add language selector dropdown in nav bar (desktop + mobile)
- Create API endpoints for language preference (POST/GET /api/i18n/)
- Support language detection: user profile > cookie > Accept-Language > default

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/views/base.py

@@ -12,6 +12,14 @@ from sqlalchemy.orm import Session  # noqa: F401
 from app.auth import require_login  # noqa: F401
 from app.config import settings
 from app.database import get_db  # noqa: F401
+from app.utils.i18n import (
+    SUPPORTED_LANGUAGES,
+    detect_language,
+    format_date,
+    format_datetime,
+    format_number,
+    translate,
+)
 
 # Set up Jinja2 templates
 templates_dir = Path(__file__).parent.parent.parent / "frontend" / "templates"
@@ -21,6 +29,16 @@ templates = Jinja2Templates(directory=str(templates_dir))
 templates.env.globals["min"] = min
 templates.env.globals["max"] = max
 
+# ---------------------------------------------------------------------------
+# i18n Jinja2 integration
+# ---------------------------------------------------------------------------
+# The _() function is available in every template to translate UI strings.
+# Usage: {{ _("nav.dashboard") }} or {{ _("upload.max_size", size="10 MB") }}
+# The locale is automatically resolved from the request context.
+# ---------------------------------------------------------------------------
+
+templates.env.globals["supported_languages"] = SUPPORTED_LANGUAGES
+
 # Customize Jinja2Templates to include app_version in all templates
 original_template_response = templates.TemplateResponse
 
@@ -48,8 +66,31 @@ def _inject_global_context(ctx: dict) -> None:
             session_user = req.session.get("user")
         # When auth is disabled every visitor is effectively "logged in"
         ctx.setdefault("is_logged_in", not getattr(settings, "auth_enabled", True) or session_user is not None)
+
+        # --- i18n: detect language and register template helpers ---
+        current_locale = detect_language(req)
+        ctx.setdefault("current_locale", current_locale)
+
+        def _translate(key: str, **kwargs: object) -> str:
+            return translate(key, current_locale, **kwargs)
+
+        def _format_date(value: object, short: bool = False) -> str:
+            return format_date(value, current_locale, short=short)  # type: ignore[arg-type]
+
+        def _format_datetime(value: object) -> str:
+            return format_datetime(value, current_locale)  # type: ignore[arg-type]
+
+        def _format_number(value: object) -> str:
+            return format_number(value, current_locale)  # type: ignore[arg-type]
+
+        ctx.setdefault("_", _translate)
+        ctx.setdefault("format_date_l10n", _format_date)
+        ctx.setdefault("format_datetime_l10n", _format_datetime)
+        ctx.setdefault("format_number_l10n", _format_number)
     else:
         ctx.setdefault("is_logged_in", not getattr(settings, "auth_enabled", True))
+        ctx.setdefault("current_locale", "en")
+        ctx.setdefault("_", lambda key, **kw: translate(key, "en", **kw))
 
 
 def template_response_with_version(*args, **kwargs):