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>

frontend/src/app/dashboard/translate/useTranslationConfig.ts

@@ -80,10 +80,11 @@ export function useTranslationConfig(hasFile: boolean): UseTranslationConfigRetu
     }
   }, [settings.defaultTargetLanguage]); // eslint-disable-line react-hooks/exhaustive-deps
 
-  // Reset glossary selection when source or target language changes
+  // Reset glossary selection when source language changes
+  // (multilingual glossaries are compatible with any target language)
   useEffect(() => {
     setGlossaryId(null);
-  }, [sourceLang, targetLang]); // eslint-disable-line react-hooks/exhaustive-deps
+  }, [sourceLang]); // eslint-disable-line react-hooks/exhaustive-deps
 
   // Fetch available (admin-configured) providers
   useEffect(() => {