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>

docs/InternationalizationGuide.md

@@ -252,8 +252,11 @@ format_number(1234.56, "en")                # → "1,234.56"
 ### Adding a New Translation Key
 
 1. Add the key and English text to `frontend/translations/en.json`
-2. Add translations for all other languages in their respective files
-3. Use `{{ _("your.new.key") }}` in templates
+2. Use `{{ _("your.new.key") }}` in templates or `translate("your.new.key", locale)` in Python
+
+That's it. An external automation script picks up new keys in `en.json` and propagates
+translations to all other language files. You never need to touch the non-English JSON
+files manually — the translate-and-sync pipeline takes care of it.
 
 ### AI Fallback Translation