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/views/base.py

@@ -35,9 +35,12 @@ templates.env.globals["max"] = max
 # 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.
+# A default English implementation is registered as a global so error handlers
+# that don't go through _inject_global_context still have the function available.
 # ---------------------------------------------------------------------------
 
 templates.env.globals["supported_languages"] = SUPPORTED_LANGUAGES
+templates.env.globals["_"] = lambda key, **kwargs: translate(key, "en", **kwargs)
 
 # Customize Jinja2Templates to include app_version in all templates
 original_template_response = templates.TemplateResponse