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>

Caddyfile

@@ -1,7 +1,27 @@
 pkm.hyungi.net {
-    # Phase 0~3: FastAPI가 모든 요청 처리 (셋업 위자드 포함)
-    # Phase 4: 프론트엔드 분리 시 경로별 라우팅 추가
-    reverse_proxy fastapi:8000
+    encode gzip
+
+    # API + 문서 → FastAPI
+    handle /api/* {
+        reverse_proxy fastapi:8000
+    }
+    handle /docs {
+        reverse_proxy fastapi:8000
+    }
+    handle /openapi.json {
+        reverse_proxy fastapi:8000
+    }
+    handle /health {
+        reverse_proxy fastapi:8000
+    }
+    handle /setup {
+        reverse_proxy fastapi:8000
+    }
+
+    # 프론트엔드
+    handle {
+        reverse_proxy frontend:3000
+    }
 }
 
 # Synology Office 프록시