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-backend.cjs

@@ -0,0 +1,40 @@
+const fs = require("fs");
+const path = require("path");
+const { spawnSync } = require("child_process");
+
+const repoRoot = path.resolve(__dirname, "..", "..");
+const entry = path.join(repoRoot, "src", "wechat_decrypt_tool", "backend_entry.py");
+
+const distDir = path.join(repoRoot, "desktop", "resources", "backend");
+const workDir = path.join(repoRoot, "desktop", "build", "pyinstaller");
+const specDir = path.join(repoRoot, "desktop", "build", "pyinstaller-spec");
+
+fs.mkdirSync(distDir, { recursive: true });
+fs.mkdirSync(workDir, { recursive: true });
+fs.mkdirSync(specDir, { recursive: true });
+
+const nativeDir = path.join(repoRoot, "src", "wechat_decrypt_tool", "native");
+const addData = `${nativeDir};wechat_decrypt_tool/native`;
+
+const args = [
+  "run",
+  "pyinstaller",
+  "--noconfirm",
+  "--clean",
+  "--name",
+  "wechat-backend",
+  "--onefile",
+  "--distpath",
+  distDir,
+  "--workpath",
+  workDir,
+  "--specpath",
+  specDir,
+  "--add-data",
+  addData,
+  entry,
+];
+
+const r = spawnSync("uv", args, { cwd: repoRoot, stdio: "inherit" });
+process.exit(r.status ?? 1);
+