feat: implement Phase 4 SvelteKit frontend + backend enhancements

Backend:
- Add dashboard API (today stats, inbox count, law alerts, pipeline status)
- Add /api/documents/tree endpoint for sidebar domain/sub_group tree
- Migrate auth to HttpOnly cookie for refresh token (XSS defense)
- Add /api/auth/logout endpoint (cookie cleanup)
- Register dashboard router in main.py

Frontend (SvelteKit + Tailwind CSS v4):
- api.ts: fetch wrapper with refresh queue pattern, 401 single retry,
  forced logout on refresh failure
- Auth store: login/logout/refresh with memory-based access token
- UI store: toast system, sidebar state
- Login page with TOTP support
- Dashboard with 4 stat widgets + recent documents
- Document list with hybrid search (debounce, URL query state, mode select)
- Document detail with format-aware viewer (markdown/PDF/HWP/Synology/fallback)
- Metadata panel (AI summary, tags, processing history)
- Inbox triage UI (batch select, confirm dialog, domain override)
- Settings page (password change, TOTP status)

Infrastructure:
- Enable frontend service in docker-compose
- Caddy path routing (/api/* → fastapi, / → frontend) + gzip

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -51,14 +51,13 @@ services:
       - KORDOC_ENDPOINT=http://kordoc-service:3100
     restart: unless-stopped
 
-  # frontend: Phase 4에서 SvelteKit 빌드 안정화 후 활성화
-  # frontend:
-  #   build: ./frontend
-  #   ports:
-  #     - "3000:3000"
-  #   depends_on:
-  #     - fastapi
-  #   restart: unless-stopped
+  frontend:
+    build: ./frontend
+    ports:
+      - "3000:3000"
+    depends_on:
+      - fastapi
+    restart: unless-stopped
 
   caddy:
     image: caddy:2
@@ -70,6 +69,7 @@ services:
       - caddy_data:/data
     depends_on:
       - fastapi
+      - frontend
     restart: unless-stopped
 
 volumes: