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>

.github/copilot-instructions.md

@@ -163,6 +163,13 @@ pytest --tb=short -q
 - Keep JavaScript minimal - prefer server-side rendering
 - Follow existing template structure and patterns
 
+### Internationalization (i18n) & Localization (l10n)
+- **Always** use the `_("key")` helper in Jinja2 templates and `translate("key", locale)` in Python for every user-visible string — never hardcode UI text.
+- **Only add new keys to `frontend/translations/en.json`** — that is the one and only file you must touch when introducing new UI strings.
+- Do **not** manually edit any non-English translation file (`de.json`, `fr.json`, etc.). An external automation script syncs all other language files from `en.json` automatically.
+- Key naming convention: `<section>.<descriptor>` in snake_case, e.g. `language.search_placeholder`, `nav.help`, `common.cancel`.
+- The `test_all_languages_have_same_keys` check has been intentionally removed — key completeness across locales is enforced by the external sync script, not by the test suite.
+
 ### Testing
 - Write tests in `tests/` directory, mirroring `app/` structure
 - Use pytest markers: `@pytest.mark.unit`, `@pytest.mark.integration`, etc.