feat(desktop): 新增 Electron 桌面端壳与自绘标题栏

- 新增 desktop/ Electron 工程：启动后端并等待 /api/health，就绪后加载页面；打包模式从 extraResources 读取 UI/后端
- 新增 DesktopTitleBar 组件，适配 frame:false 自绘标题栏，并修复桌面端 100vh 布局导致的外层滚动条
- chat 页面右侧布局调整更接近原生微信；detection-result 调试输出仅在 dev 环境启用
- .gitignore 忽略 desktop 构建产物/依赖，保留 .gitkeep 占位文件
- README 补充 Windows 桌面端 EXE 打包（npm run dist）与产物路径说明

desktop/scripts/build-icon.cjs

@@ -0,0 +1,49 @@
+const fs = require("fs");
+const path = require("path");
+
+const pngToIco = require("png-to-ico").default;
+const { PNG } = require("pngjs");
+
+const repoRoot = path.resolve(__dirname, "..", "..");
+const srcPng = path.join(repoRoot, "frontend", "public", "logo.png");
+const dstIco = path.join(repoRoot, "desktop", "resources", "icon.ico");
+
+async function main() {
+  if (!fs.existsSync(srcPng)) {
+    // eslint-disable-next-line no-console
+    console.error(`Logo not found: ${srcPng}`);
+    process.exit(1);
+  }
+
+  const raw = fs.readFileSync(srcPng);
+  const input = PNG.sync.read(raw);
+  const size = Math.max(input.width, input.height);
+
+  const square = new PNG({ width: size, height: size });
+  const dx = Math.floor((size - input.width) / 2);
+  const dy = Math.floor((size - input.height) / 2);
+  for (let y = 0; y < input.height; y += 1) {
+    const srcStart = y * input.width * 4;
+    const srcEnd = srcStart + input.width * 4;
+    const dstStart = ((y + dy) * size + dx) * 4;
+    input.data.copy(square.data, dstStart, srcStart, srcEnd);
+  }
+
+  const tmpDir = path.join(repoRoot, "desktop", "build", "icon");
+  fs.mkdirSync(tmpDir, { recursive: true });
+  const tmpPng = path.join(tmpDir, "logo-square.png");
+  fs.writeFileSync(tmpPng, PNG.sync.write(square));
+
+  const buf = await pngToIco(tmpPng);
+  fs.mkdirSync(path.dirname(dstIco), { recursive: true });
+  fs.writeFileSync(dstIco, buf);
+
+  // eslint-disable-next-line no-console
+  console.log(`Generated icon: ${dstIco}`);
+}
+
+main().catch((err) => {
+  // eslint-disable-next-line no-console
+  console.error(err);
+  process.exit(1);
+});