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/api.ts

@@ -0,0 +1,133 @@
+/**
+ * API fetch 래퍼
+ *
+ * - access token: 메모리 변수
+ * - refresh token: HttpOnly cookie (서버가 관리)
+ * - refresh 중복 방지: isRefreshing 플래그 + 대기 큐
+ * - 401 retry: 1회만, 실패 시 강제 logout
+ */
+
+const API_BASE = '/api';
+
+let accessToken: string | null = null;
+
+// refresh 큐
+let isRefreshing = false;
+let refreshQueue: Array<{
+  resolve: (token: string) => void;
+  reject: (err: Error) => void;
+}> = [];
+
+export function setAccessToken(token: string | null) {
+  accessToken = token;
+}
+
+export function getAccessToken(): string | null {
+  return accessToken;
+}
+
+async function refreshAccessToken(): Promise<string> {
+  const res = await fetch(`${API_BASE}/auth/refresh`, {
+    method: 'POST',
+    credentials: 'include', // cookie 전송
+  });
+  if (!res.ok) {
+    throw new Error('refresh failed');
+  }
+  const data = await res.json();
+  accessToken = data.access_token;
+  return data.access_token;
+}
+
+function processRefreshQueue(error: Error | null, token: string | null) {
+  refreshQueue.forEach(({ resolve, reject }) => {
+    if (error) reject(error);
+    else resolve(token!);
+  });
+  refreshQueue = [];
+}
+
+async function handleTokenRefresh(): Promise<string> {
+  if (isRefreshing) {
+    return new Promise((resolve, reject) => {
+      refreshQueue.push({ resolve, reject });
+    });
+  }
+
+  isRefreshing = true;
+  try {
+    const token = await refreshAccessToken();
+    processRefreshQueue(null, token);
+    return token;
+  } catch (err) {
+    const error = err instanceof Error ? err : new Error('refresh failed');
+    processRefreshQueue(error, null);
+    // 강제 logout
+    accessToken = null;
+    if (typeof window !== 'undefined') {
+      window.location.href = '/login';
+    }
+    throw error;
+  } finally {
+    isRefreshing = false;
+  }
+}
+
+export type ApiError = {
+  status: number;
+  detail: string;
+};
+
+export async function api<T = unknown>(
+  path: string,
+  options: RequestInit = {},
+): Promise<T> {
+  const headers: Record<string, string> = {
+    ...(options.headers as Record<string, string> || {}),
+  };
+
+  if (accessToken) {
+    headers['Authorization'] = `Bearer ${accessToken}`;
+  }
+
+  // FormData일 때는 Content-Type 자동 설정
+  if (options.body && !(options.body instanceof FormData)) {
+    headers['Content-Type'] = 'application/json';
+  }
+
+  const res = await fetch(`${API_BASE}${path}`, {
+    ...options,
+    headers,
+    credentials: 'include',
+  });
+
+  // 401 → refresh 1회 시도
+  if (res.status === 401 && accessToken) {
+    try {
+      await handleTokenRefresh();
+      headers['Authorization'] = `Bearer ${accessToken}`;
+      const retryRes = await fetch(`${API_BASE}${path}`, {
+        ...options,
+        headers,
+        credentials: 'include',
+      });
+      if (!retryRes.ok) {
+        const err = await retryRes.json().catch(() => ({ detail: 'Unknown error' }));
+        throw { status: retryRes.status, detail: err.detail || retryRes.statusText };
+      }
+      return retryRes.json();
+    } catch {
+      throw { status: 401, detail: '인증이 만료되었습니다' };
+    }
+  }
+
+  if (!res.ok) {
+    const err = await res.json().catch(() => ({ detail: res.statusText }));
+    throw { status: res.status, detail: err.detail || res.statusText } as ApiError;
+  }
+
+  // 204 No Content
+  if (res.status === 204) return {} as T;
+
+  return res.json();
+}