fix: resolve all 47 failing tests in main

- Fix detect_language() to safely handle unhashable session values and
  requests missing cookies/headers attributes (TypeError + AttributeError)
- Add default English `_()` translation function to Jinja2 template
  environment globals so error pages always have it available
- Fix app/main.py exception handlers to use a dedicated error templates
  instance with `_` registered, keeping it separate from view templates
  to avoid test patches breaking error rendering
- Fix app/views/plans.py to import shared templates from app.views.base
  instead of creating its own Jinja2Templates instance
- Make migration 029_add_user_language_preference idempotent: skip
  ALTER TABLE if user_profiles table does not exist
- Update test_i18n.py expectations to reflect 31 supported languages
- Create 21 missing translation files (nb, da, sv, fi, is, ga, lb, ca,
  cs, sk, hu, sl, hr, ro, bg, el, et, lv, lt, tr, uk) with English
  placeholder translations
- Update de.json with 117 missing translation keys including proper
  German translations
- Update es, fr, it, nl, pl, pt, ru, zh translation files with missing
  keys using English fallbacks

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

app/main.py

@@ -239,6 +239,21 @@ else:
 
 
 # Custom exception handlers that return JSON for API routes and HTML for frontend routes
+# These use their own separate templates instance so that patches in tests on individual
+# view modules do not affect the error handler rendering.
+_error_templates_dir = pathlib.Path(__file__).parents[1] / "frontend" / "templates"
+_error_templates = Jinja2Templates(directory=str(_error_templates_dir))
+# Register the i18n translate helper as a global so error templates can use {{ _("key") }}.
+# Error pages use the default language (English); request-specific locale is not needed here.
+from app.utils.i18n import SUPPORTED_LANGUAGES as _SUPPORTED_LANGUAGES  # noqa: E402
+from app.utils.i18n import translate as _translate_fn  # noqa: E402
+
+_error_templates.env.globals["_"] = lambda key, **kwargs: _translate_fn(key, "en", **kwargs)
+_error_templates.env.globals["min"] = min
+_error_templates.env.globals["max"] = max
+_error_templates.env.globals["supported_languages"] = _SUPPORTED_LANGUAGES
+
+
 @app.exception_handler(HTTPException)
 async def http_exception_handler(request: Request, exc: HTTPException):
     """
@@ -250,15 +265,15 @@ async def http_exception_handler(request: Request, exc: HTTPException):
         return JSONResponse(status_code=exc.status_code, content={"detail": exc.detail})
 
     # For frontend routes, return appropriate HTML templates
-    templates = Jinja2Templates(directory=str(static_dir.parent / "templates"))
-
     # Handle 404 errors with a custom template
     if exc.status_code == 404:
-        return templates.TemplateResponse("404.html", {"request": request}, status_code=status.HTTP_404_NOT_FOUND)
+        return _error_templates.TemplateResponse(
+            "404.html", {"request": request}, status_code=status.HTTP_404_NOT_FOUND
+        )
 
     # For other HTTP errors, we could create specific templates or use a generic one
     # For now, return a simple error page
-    return templates.TemplateResponse(
+    return _error_templates.TemplateResponse(
         "404.html",  # Reuse 404 template for other errors, or create a generic error template
         {"request": request},
         status_code=exc.status_code,
@@ -279,8 +294,7 @@ async def custom_500_handler(request: Request, exc: Exception):
         )
 
     # Serve the 500 template for non-API routes
-    templates = Jinja2Templates(directory=str(static_dir.parent / "templates"))
-    return templates.TemplateResponse(
+    return _error_templates.TemplateResponse(
         "500.html",
         {"request": request, "exc": exc},
         status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,