AGC 更新提示增加 X 关闭按钮 (#276)

## 变更内容
- 为 AGC 更新提示增加可访问的 X 关闭按钮。
- 复用现有 dismiss 逻辑，手动关闭后立即移除提示。
- 增加关闭按钮的 hover、禁用和图标样式。

## 验证
- npm --prefix apps/ai-game-creator-shell run typecheck
- npm exec vitest run apps/ai-game-creator-shell/tests/appUpdate.test.ts
- npm run check:encoding
- git diff --check

Reviewed-on: http://192.168.35.82/git/GenarrativeAI/Genarrative/pulls/276
Reviewed-by: 段舒康 <kdletters@qq.com>
Co-authored-by: 孔令弘 <ink29535@proton.me>
Co-committed-by: 孔令弘 <ink29535@proton.me>

apps/ai-game-creator-shell/src/components/AppUpdateNotice.tsx

@@ -1,5 +1,5 @@
 import { listen } from '@tauri-apps/api/event';
-import { Download, LoaderCircle } from 'lucide-react';
+import { Download, LoaderCircle, X } from 'lucide-react';
 import { useEffect, useState } from 'react';
 
 import { APP_VERSION } from '../app/appMetadata';
@@ -117,6 +117,16 @@ export function AppUpdateNotice() {
         >
           {isDownloading ? '正在下载…' : '下载更新'}
         </button>
+        <button
+          type="button"
+          className="app-update-close"
+          onClick={dismiss}
+          disabled={isDownloading}
+          aria-label="关闭更新提示"
+          title="关闭"
+        >
+          <X size={18} aria-hidden="true" />
+        </button>
       </aside>
       {downloadState !== 'idle' ? (
         <div

apps/ai-game-creator-shell/src/styles.css

@@ -75,6 +75,27 @@ body {
   cursor: wait;
   opacity: 0.65;
 }
+.app-update-notice .app-update-close {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  width: 28px;
+  height: 28px;
+  padding: 0;
+  border: 0;
+  border-radius: 6px;
+  background: transparent;
+  color: #8d6a58;
+  cursor: pointer;
+}
+.app-update-notice .app-update-close:hover {
+  background: rgb(199 101 61 / 12%);
+  color: #4a220f;
+}
+.app-update-notice .app-update-close:disabled {
+  cursor: wait;
+  opacity: 0.5;
+}
 
 .app-update-overlay {
   position: fixed;