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/lib/stores/ui.ts

@@ -0,0 +1,27 @@
+import { writable } from 'svelte/store';
+
+export const sidebarOpen = writable(true);
+export const selectedDocId = writable<number | null>(null);
+
+// Toast 시스템
+interface Toast {
+  id: number;
+  type: 'success' | 'error' | 'warning' | 'info';
+  message: string;
+}
+
+let toastId = 0;
+export const toasts = writable<Toast[]>([]);
+
+export function addToast(type: Toast['type'], message: string, duration = 5000) {
+  const id = ++toastId;
+  toasts.update(t => [...t, { id, type, message }]);
+  if (duration > 0) {
+    setTimeout(() => removeToast(id), duration);
+  }
+  return id;
+}
+
+export function removeToast(id: number) {
+  toasts.update(t => t.filter(toast => toast.id !== id));
+}