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>

tests/test_i18n.py

@@ -326,11 +326,43 @@ class TestSupportedLanguages:
 
     @pytest.mark.unit
     def test_ten_languages_supported(self) -> None:
-        assert len(SUPPORTED_LANGUAGES) == 10
+        assert len(SUPPORTED_LANGUAGES) == 31
 
     @pytest.mark.unit
     def test_supported_codes_set(self) -> None:
-        expected = {"en", "de", "fr", "es", "it", "pt", "nl", "pl", "zh", "ru"}
+        expected = {
+            "en",
+            "de",
+            "fr",
+            "es",
+            "it",
+            "pt",
+            "nl",
+            "pl",
+            "zh",
+            "ru",
+            "nb",
+            "da",
+            "sv",
+            "fi",
+            "is",
+            "ga",
+            "lb",
+            "ca",
+            "cs",
+            "sk",
+            "hu",
+            "sl",
+            "hr",
+            "ro",
+            "bg",
+            "el",
+            "et",
+            "lv",
+            "lt",
+            "tr",
+            "uk",
+        }
         assert SUPPORTED_LANGUAGE_CODES == expected
 
     @pytest.mark.unit
@@ -353,7 +385,7 @@ class TestI18nAPI:
         assert response.status_code == 200
         data = response.json()
         assert "languages" in data
-        assert len(data["languages"]) == 10
+        assert len(data["languages"]) == 31
         assert data["default"] == "en"
         # Verify each language has required fields
         for lang in data["languages"]: