feat(web): 添加响应式布局和 PWA 支持

- 实现移动端响应式适配：抽屉式侧边栏、触摸优化、Safe Area 支持
- 添加 PWA 支持：vite-plugin-pwa、manifest、Service Worker 缓存
- 优化移动端输入体验：防缩放、最小触摸目标、键盘适配

packages/web/public/icon.svg

@@ -0,0 +1,22 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512">
+  <defs>
+    <linearGradient id="grad" x1="0%" y1="0%" x2="100%" y2="100%">
+      <stop offset="0%" style="stop-color:#0ea5e9;stop-opacity:1" />
+      <stop offset="100%" style="stop-color:#0369a1;stop-opacity:1" />
+    </linearGradient>
+  </defs>
+  <!-- Background -->
+  <rect width="512" height="512" rx="96" fill="#111827"/>
+  <!-- Terminal icon -->
+  <g fill="none" stroke="url(#grad)" stroke-width="28" stroke-linecap="round" stroke-linejoin="round">
+    <!-- Terminal window -->
+    <rect x="96" y="128" width="320" height="256" rx="24" fill="none"/>
+    <!-- Command prompt > -->
+    <polyline points="144,224 192,272 144,320"/>
+    <!-- Cursor line -->
+    <line x1="224" y1="320" x2="320" y2="320"/>
+  </g>
+  <!-- AI sparkle -->
+  <circle cx="384" cy="160" r="8" fill="#0ea5e9"/>
+  <circle cx="400" cy="176" r="4" fill="#38bdf8"/>
+</svg>

packages/web/public/manifest.json

@@ -0,0 +1,19 @@
+{
+  "name": "AI Terminal Assistant",
+  "short_name": "AI Assistant",
+  "description": "Your intelligent coding companion",
+  "theme_color": "#111827",
+  "background_color": "#111827",
+  "display": "standalone",
+  "orientation": "portrait-primary",
+  "scope": "/",
+  "start_url": "/",
+  "icons": [
+    {
+      "src": "/icon.svg",
+      "sizes": "any",
+      "type": "image/svg+xml",
+      "purpose": "any maskable"
+    }
+  ]
+}