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/settings/+page.svelte

@@ -0,0 +1,89 @@
+<script>
+  import { api } from '$lib/api';
+  import { addToast } from '$lib/stores/ui';
+  import { user } from '$lib/stores/auth';
+
+  let currentPassword = '';
+  let newPassword = '';
+  let confirmPassword = '';
+  let changing = false;
+
+  async function changePassword() {
+    if (newPassword !== confirmPassword) {
+      addToast('error', '새 비밀번호가 일치하지 않습니다');
+      return;
+    }
+    if (newPassword.length < 8) {
+      addToast('error', '비밀번호는 8자 이상이어야 합니다');
+      return;
+    }
+
+    changing = true;
+    try {
+      await api('/auth/change-password', {
+        method: 'POST',
+        body: JSON.stringify({
+          current_password: currentPassword,
+          new_password: newPassword,
+        }),
+      });
+      addToast('success', '비밀번호가 변경되었습니다');
+      currentPassword = '';
+      newPassword = '';
+      confirmPassword = '';
+    } catch (err) {
+      addToast('error', err.detail || '비밀번호 변경 실패');
+    } finally {
+      changing = false;
+    }
+  }
+</script>
+
+<div class="min-h-screen">
+  <nav class="flex items-center gap-2 px-6 py-3 border-b border-[var(--border)] bg-[var(--surface)] text-sm">
+    <a href="/" class="text-[var(--text-dim)] hover:text-[var(--text)]">PKM</a>
+    <span class="text-[var(--text-dim)]">/</span>
+    <span>설정</span>
+  </nav>
+
+  <div class="max-w-lg mx-auto p-6">
+    <!-- 계정 정보 -->
+    <div class="bg-[var(--surface)] rounded-xl border border-[var(--border)] p-5 mb-6">
+      <h2 class="text-lg font-semibold mb-3">계정 정보</h2>
+      <dl class="space-y-2 text-sm">
+        <div class="flex justify-between">
+          <dt class="text-[var(--text-dim)]">아이디</dt>
+          <dd>{$user?.username}</dd>
+        </div>
+        <div class="flex justify-between">
+          <dt class="text-[var(--text-dim)]">2FA (TOTP)</dt>
+          <dd class={$user?.totp_enabled ? 'text-[var(--success)]' : 'text-[var(--text-dim)]'}>
+            {$user?.totp_enabled ? '활성' : '비활성'}
+          </dd>
+        </div>
+      </dl>
+    </div>
+
+    <!-- 비밀번호 변경 -->
+    <div class="bg-[var(--surface)] rounded-xl border border-[var(--border)] p-5">
+      <h2 class="text-lg font-semibold mb-3">비밀번호 변경</h2>
+      <form onsubmit={(e) => { e.preventDefault(); changePassword(); }} class="space-y-3">
+        <div>
+          <label class="block text-sm text-[var(--text-dim)] mb-1">현재 비밀번호</label>
+          <input type="password" bind:value={currentPassword} class="w-full px-3 py-2 bg-[var(--bg)] border border-[var(--border)] rounded-lg text-[var(--text)] outline-none focus:border-[var(--accent)]" />
+        </div>
+        <div>
+          <label class="block text-sm text-[var(--text-dim)] mb-1">새 비밀번호</label>
+          <input type="password" bind:value={newPassword} class="w-full px-3 py-2 bg-[var(--bg)] border border-[var(--border)] rounded-lg text-[var(--text)] outline-none focus:border-[var(--accent)]" />
+        </div>
+        <div>
+          <label class="block text-sm text-[var(--text-dim)] mb-1">새 비밀번호 확인</label>
+          <input type="password" bind:value={confirmPassword} class="w-full px-3 py-2 bg-[var(--bg)] border border-[var(--border)] rounded-lg text-[var(--text)] outline-none focus:border-[var(--accent)]" />
+        </div>
+        <button type="submit" disabled={changing} class="w-full py-2.5 bg-[var(--accent)] text-white rounded-lg disabled:opacity-50">
+          {changing ? '변경 중...' : '비밀번호 변경'}
+        </button>
+      </form>
+    </div>
+  </div>
+</div>