feat: add web entry to download desktop client

frontend/app.html

@@ -1796,6 +1796,10 @@
           {{ isLogin ? '还没有账号？' : '已有账号？' }}
           <a @click="toggleAuthMode">{{ isLogin ? '立即注册' : '去登录' }}</a>
         </div>
+        <button type="button" class="btn btn-secondary" style="margin-top: 12px;" :disabled="desktopClientDownloading" @click="downloadDesktopClient">
+          <i :class="desktopClientDownloading ? 'fas fa-spinner fa-spin' : 'fas fa-desktop'"></i>
+          {{ desktopClientDownloading ? '获取安装包...' : '下载桌面客户端' }}
+        </button>
       </div>
     </div>
     <!-- 导航栏 -->
@@ -1818,6 +1822,10 @@
 
           <i class="fas fa-cog"></i> 设置
         </div>
+        <button class="btn btn-secondary" :disabled="desktopClientDownloading" @click="downloadDesktopClient">
+          <i :class="desktopClientDownloading ? 'fas fa-spinner fa-spin' : 'fas fa-download'"></i>
+          {{ desktopClientDownloading ? '准备中...' : '下载客户端' }}
+        </button>
         <div class="user-info">
           <i class="fas fa-user-circle"></i>
           <span>{{ user.username }}</span>
@@ -2480,6 +2488,21 @@
           <p class="settings-page-subtitle">管理存储策略、主题外观与账号安全</p>
         </div>
 
+        <h3 class="settings-section-title" style="margin-bottom: 20px;">
+          <i class="fas fa-desktop"></i> 客户端下载
+        </h3>
+        <div class="settings-panel" style="background: var(--bg-card); padding: 20px; border-radius: 12px; border: 1px solid var(--glass-border); margin-bottom: 30px;">
+          <div style="display: flex; justify-content: space-between; align-items: center; gap: 12px; flex-wrap: wrap;">
+            <div style="color: var(--text-secondary); font-size: 13px;">
+              下载最新版桌面客户端（Windows x64）
+            </div>
+            <button class="btn btn-primary" :disabled="desktopClientDownloading" @click="downloadDesktopClient">
+              <i :class="desktopClientDownloading ? 'fas fa-spinner fa-spin' : 'fas fa-download'"></i>
+              {{ desktopClientDownloading ? '获取安装包...' : '下载桌面客户端' }}
+            </button>
+          </div>
+        </div>
+
         <!-- 存储管理 - 仅用户可选择 -->
         <div v-if="user && !user.is_admin && storagePermission === 'user_choice'" class="settings-section settings-storage-choice" style="margin-bottom: 40px;">
           <h3 class="settings-section-title" style="margin-bottom: 20px;">