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/utils/i18n.py

@@ -223,16 +223,19 @@ def detect_language(request: Request) -> str:
     # 1. User session preference
     if hasattr(request, "session"):
         session_lang = request.session.get("preferred_language")
-        if session_lang and session_lang in SUPPORTED_LANGUAGE_CODES:
+        if isinstance(session_lang, str) and session_lang in SUPPORTED_LANGUAGE_CODES:
             return session_lang
 
     # 2. Cookie
-    cookie_lang = request.cookies.get("docuelevate_lang")
-    if cookie_lang and cookie_lang in SUPPORTED_LANGUAGE_CODES:
-        return cookie_lang
+    if hasattr(request, "cookies"):
+        cookie_lang = request.cookies.get("docuelevate_lang")
+        if isinstance(cookie_lang, str) and cookie_lang in SUPPORTED_LANGUAGE_CODES:
+            return cookie_lang
 
     # 3. Accept-Language header
-    accept = request.headers.get("accept-language", "")
+    accept = ""
+    if hasattr(request, "headers"):
+        accept = request.headers.get("accept-language", "")
     lang = _parse_accept_language(accept)
     if lang:
         return lang