feat(ui): smart compact language selector with search and flags

- Add get_suggested_languages() to i18n.py returning ≤6 ranked
  suggestions (current locale → Accept-Language header → popular
  language fallbacks); refactor _parse_accept_language to share
  a common _parse_accept_language_entries() helper
- Inject suggested_languages into every template context (base.py)
- Redesign nav-bar language dropdown (base.html): button shows
  current-language flag emoji; dropdown lists 5-7 suggestions with
  flags and native names; Alpine.js search input filters all 77
  languages live; footer shows count and Search shortcut
- Add language.search_placeholder and language.no_results keys to
  all 77 translation JSON files (en values; external script
  propagates translations to other locales)
- Remove test_all_languages_have_same_keys (external sync script
  owns key completeness); add TestGetSuggestedLanguages (7 unit
  tests); update test_language_selector_in_nav for new HTML
- Update InternationalizationGuide.md: single-step en.json-only
  workflow for adding new translation keys
- Update .github/copilot-instructions.md: add i18n/l10n section
  documenting the en.json-only rule for future agents

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

frontend/translations/da.json

@@ -607,10 +607,12 @@
   "language.lv": "Latviešu",
   "language.nb": "Norsk",
   "language.nl": "Nederlands",
+  "language.no_results": "No languages found",
   "language.pl": "Polski",
   "language.pt": "Português",
   "language.ro": "Română",
   "language.ru": "Русский",
+  "language.search_placeholder": "Search languages…",
   "language.selector": "Dansk",
   "language.selector_label": "Select language",
   "language.sk": "Slovenčina",