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>

migrations/versions/029_add_user_language_preference.py

@@ -17,12 +17,24 @@ depends_on: Union[str, None] = None
 
 def upgrade() -> None:
     """Add preferred_language column to user_profiles table."""
-    op.add_column(
-        "user_profiles",
-        sa.Column("preferred_language", sa.String(10), nullable=True, server_default=None),
-    )
+    conn = op.get_bind()
+    inspector = sa.inspect(conn)
+    if "user_profiles" not in inspector.get_table_names():
+        return
+    existing_columns = {col["name"] for col in inspector.get_columns("user_profiles")}
+    if "preferred_language" not in existing_columns:
+        op.add_column(
+            "user_profiles",
+            sa.Column("preferred_language", sa.String(10), nullable=True, server_default=None),
+        )
 
 
 def downgrade() -> None:
     """Remove preferred_language column from user_profiles table."""
-    op.drop_column("user_profiles", "preferred_language")
+    conn = op.get_bind()
+    inspector = sa.inspect(conn)
+    if "user_profiles" not in inspector.get_table_names():
+        return
+    existing_columns = {col["name"] for col in inspector.get_columns("user_profiles")}
+    if "preferred_language" in existing_columns:
+        op.drop_column("user_profiles", "preferred_language")