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/routes/+page.svelte

@@ -1,14 +1,107 @@
 <script>
-  // TODO: Phase 4에서 대시보드 위젯 구현
+  import { onMount } from 'svelte';
+  import { goto } from '$app/navigation';
+  import { api } from '$lib/api';
+  import { isAuthenticated, user, logout } from '$lib/stores/auth';
+  import { sidebarOpen, addToast } from '$lib/stores/ui';
+
+  let dashboard = null;
+  let loading = true;
+
+  onMount(async () => {
+    try {
+      dashboard = await api('/dashboard/');
+    } catch (err) {
+      addToast('error', '대시보드 로딩 실패');
+    } finally {
+      loading = false;
+    }
+  });
 </script>
 
-<h1>hyungi Document Server</h1>
-<p>PKM 대시보드 — Phase 4에서 구현 예정</p>
+<div class="min-h-screen">
+  <!-- 상단 네비게이션 -->
+  <nav class="flex items-center justify-between px-6 py-3 border-b border-[var(--border)] bg-[var(--surface)]">
+    <div class="flex items-center gap-4">
+      <button onclick={() => sidebarOpen.update(v => !v)} class="text-[var(--text-dim)] hover:text-[var(--text)]">
+        &#9776;
+      </button>
+      <h1 class="text-lg font-semibold">hyungi Document Server</h1>
+    </div>
+    <div class="flex items-center gap-4">
+      <a href="/documents" class="text-sm text-[var(--text-dim)] hover:text-[var(--text)]">문서</a>
+      <a href="/inbox" class="text-sm text-[var(--text-dim)] hover:text-[var(--text)]">Inbox</a>
+      <span class="text-sm text-[var(--text-dim)]">{$user?.username}</span>
+      <button onclick={() => { logout(); goto('/login'); }} class="text-sm text-[var(--error)] hover:underline">로그아웃</button>
+    </div>
+  </nav>
 
-<section>
-  <h2>시스템 상태</h2>
-  <ul>
-    <li>FastAPI: <a href="/api/health">헬스체크</a></li>
-    <li>API 문서: <a href="/docs">OpenAPI</a></li>
-  </ul>
-</section>
+  <!-- 대시보드 -->
+  <div class="max-w-6xl mx-auto p-6">
+    <h2 class="text-xl font-bold mb-6">대시보드</h2>
+
+    {#if loading}
+      <div class="grid grid-cols-1 md:grid-cols-2 lg:grid-cols-4 gap-4">
+        {#each Array(4) as _}
+          <div class="bg-[var(--surface)] rounded-xl p-5 border border-[var(--border)] animate-pulse h-28"></div>
+        {/each}
+      </div>
+    {:else if dashboard}
+      <!-- 위젯 그리드 -->
+      <div class="grid grid-cols-1 md:grid-cols-2 lg:grid-cols-4 gap-4 mb-8">
+        <!-- 전체 문서 -->
+        <div class="bg-[var(--surface)] rounded-xl p-5 border border-[var(--border)]">
+          <p class="text-sm text-[var(--text-dim)]">전체 문서</p>
+          <p class="text-3xl font-bold mt-1">{dashboard.total_documents}</p>
+          <p class="text-xs text-[var(--text-dim)] mt-1">오늘 +{dashboard.today_added}</p>
+        </div>
+
+        <!-- Inbox -->
+        <div class="bg-[var(--surface)] rounded-xl p-5 border border-[var(--border)]">
+          <p class="text-sm text-[var(--text-dim)]">Inbox 미분류</p>
+          <p class="text-3xl font-bold mt-1" class:text-[var(--warning)]={dashboard.inbox_count > 0}>{dashboard.inbox_count}</p>
+          {#if dashboard.inbox_count > 0}
+            <a href="/inbox" class="text-xs text-[var(--accent)] hover:underline mt-1 inline-block">분류하기</a>
+          {/if}
+        </div>
+
+        <!-- 법령 알림 -->
+        <div class="bg-[var(--surface)] rounded-xl p-5 border border-[var(--border)]">
+          <p class="text-sm text-[var(--text-dim)]">법령 알림</p>
+          <p class="text-3xl font-bold mt-1">{dashboard.law_alerts}</p>
+          <p class="text-xs text-[var(--text-dim)] mt-1">오늘 변경</p>
+        </div>
+
+        <!-- 파이프라인 -->
+        <div class="bg-[var(--surface)] rounded-xl p-5 border border-[var(--border)]">
+          <p class="text-sm text-[var(--text-dim)]">파이프라인</p>
+          {#if dashboard.failed_count > 0}
+            <p class="text-3xl font-bold mt-1 text-[var(--error)]">{dashboard.failed_count} 실패</p>
+          {:else}
+            <p class="text-3xl font-bold mt-1 text-[var(--success)]">정상</p>
+          {/if}
+        </div>
+      </div>
+
+      <!-- 최근 문서 -->
+      <div class="bg-[var(--surface)] rounded-xl border border-[var(--border)] p-5">
+        <h3 class="text-sm font-semibold text-[var(--text-dim)] mb-3">최근 문서</h3>
+        {#if dashboard.recent_documents.length > 0}
+          <div class="space-y-2">
+            {#each dashboard.recent_documents as doc}
+              <a href="/documents/{doc.id}" class="flex items-center justify-between py-2 px-3 rounded-lg hover:bg-[var(--bg)] transition-colors">
+                <div class="flex items-center gap-3">
+                  <span class="text-xs px-2 py-0.5 rounded bg-[var(--border)] text-[var(--text-dim)] uppercase">{doc.file_format}</span>
+                  <span class="text-sm truncate max-w-md">{doc.title || '제목 없음'}</span>
+                </div>
+                <span class="text-xs text-[var(--text-dim)]">{doc.ai_domain || ''}</span>
+              </a>
+            {/each}
+          </div>
+        {:else}
+          <p class="text-sm text-[var(--text-dim)]">문서가 없습니다</p>
+        {/if}
+      </div>
+    {/if}
+  </div>
+</div>