fix: comprehensive glossary system audit — 6 critical fixes

Full audit found 18 issues across backend, frontend, and data files.
Root cause: target_language had no single source of truth — 'en' was
hardcoded as default in 6+ places while templates are actually multilingual.

Fixes applied:
- routes/glossary_routes.py: list_glossaries() fallback 'en' → 'multi'
- routes/glossary_routes.py: import reads target_lang from index.json
  (source of truth) instead of template file
- data/glossaries/*.json: all 8 template files target_lang 'en' → 'multi'
- services/glossary_service.py: get_glossary_terms() now returns
  target_language field (was missing entirely)
- schemas/glossary_schemas.py: all defaults 'en' → 'multi'
  (GlossaryCreate, GlossaryResponse, GlossaryListItem)
- useTranslationConfig.ts: only reset glossary on sourceLang change,
  not targetLang (multilingual glossaries work with any target)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/glossary_service.py

@@ -61,6 +61,7 @@ def get_glossary_terms(glossary_id: str, user_id: str) -> Dict[str, Any]:
 
             return {
                 "source_language": glossary.source_language or "fr",
+                "target_language": getattr(glossary, "target_language", None) or "multi",
                 "terms": result,
             }