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>

routes/glossary_routes.py

@@ -187,7 +187,7 @@ async def list_glossaries(
                     id=g.id,
                     name=g.name,
                     source_language=g.source_language or "fr",
-                    target_language=getattr(g, "target_language", None) or "en",
+                    target_language=getattr(g, "target_language", None) or "multi",
                     terms_count=len(g.terms) if g.terms else 0,
                     created_at=g.created_at,
                 )
@@ -596,8 +596,8 @@ async def import_glossary_template(
             glossary = Glossary(
                 user_id=user.id,
                 name=glossary_name,
-                source_language=template_data.get("source_lang", "fr"),
-                target_language=template_data.get("target_lang", "en"),
+                source_language=template_info.get("source_lang", template_data.get("source_lang", "fr")),
+                target_language=template_info.get("target_lang", template_data.get("target_lang", "multi")),
                 created_at=datetime.now(timezone.utc),
                 updated_at=datetime.now(timezone.utc),
             )