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>

frontend/src/app.css

@@ -0,0 +1,26 @@
+@import "tailwindcss";
+
+:root {
+  --bg: #0f1117;
+  --surface: #1a1d27;
+  --border: #2a2d3a;
+  --text: #e4e4e7;
+  --text-dim: #8b8d98;
+  --accent: #6c8aff;
+  --accent-hover: #859dff;
+  --error: #f5564e;
+  --success: #4ade80;
+  --warning: #fbbf24;
+}
+
+body {
+  background: var(--bg);
+  color: var(--text);
+  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
+  margin: 0;
+}
+
+/* 스크롤바 */
+::-webkit-scrollbar { width: 6px; }
+::-webkit-scrollbar-track { background: var(--bg); }
+::-webkit-scrollbar-thumb { background: var(--border); border-radius: 3px; }