feat(desktop,ci): GitHub Releases 分发 + 桌面端检查更新

让桌面端像开源项目一样用 GitHub Releases 分发，旧版用户自动获知更新：
- CD：.github/workflows/release.yml —— 打 vX.Y.Z 标签触发，macOS(universal)/
  Windows(amd64) 用 wails build 出安装包，softprops/action-gh-release 发布到
  GitHub Release（GOWORK=off，因 desktop 不在工作区）。
- 桌面端检查更新：lib/version.ts（APP_VERSION + 查 GitHub /releases/latest +
  语义版本比较 + openExternal 走 Wails BrowserOpenURL）；UpdateBanner 启动时
  检查，有新版顶部横幅「前往下载」。404/限流/离线均静默不打扰。

注：未签名/公证（macOS Gatekeeper / Win SmartScreen 需后续证书）；release
workflow 需真实 tag push 才能验证（GitHub 侧运行）。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

sundynix-desktop/frontend/src/App.tsx

@@ -12,6 +12,7 @@ import { RunsView } from "./views/RunsView";
 import { Home } from "./views/Home";
 import { Placeholder } from "./views/Placeholder";
 import { CommandPalette, type Command } from "./components/CommandPalette";
+import { UpdateBanner } from "./components/UpdateBanner";
 import { Login } from "./views/Login";
 import { submitTask, streamTokens, streamExec, authMe, logout, type Identity, type AuthUser } from "./lib/api";
 import type { TaskDsl } from "./lib/dsl";
@@ -158,6 +159,7 @@ export default function App() {
         className="pointer-events-none absolute inset-x-0 top-0 h-64 opacity-60"
         style={{ background: "radial-gradient(60% 100% at 50% 0%, rgba(124,92,246,0.10), transparent 70%)" }}
       />
+      <UpdateBanner />
       <TopBar user={user} onLogout={onLogout} onCommand={() => setCmdOpen(true)} />
       <div className="relative flex min-h-0 flex-1">
         <LeftNav active={view} onSelect={setView} />