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>

app/views/base.py

@@ -18,6 +18,7 @@ from app.utils.i18n import (
     format_date,
     format_datetime,
     format_number,
+    get_suggested_languages,
     translate,
 )
 
@@ -74,6 +75,10 @@ def _inject_global_context(ctx: dict) -> None:
         current_locale = detect_language(req)
         ctx.setdefault("current_locale", current_locale)
 
+        # Smart language suggestions for the compact nav-bar dropdown (5-7 languages)
+        accept_header = req.headers.get("accept-language", "") if hasattr(req, "headers") else ""
+        ctx.setdefault("suggested_languages", get_suggested_languages(current_locale, accept_header))
+
         def _translate(key: str, **kwargs: object) -> str:
             return translate(key, current_locale, **kwargs)