fix(tauri): stop stale UI assets from shadowing desktop builds (#258)

## Summary
- prefer the bundled desktop UI over the downloaded cache when both
report the same version, so rebuilt installers do not keep serving stale
frontend assets
- rebuild the server workspace during the Tauri prebuild step on every
desktop package build, matching Electron's correctness boundary for
fresh UI/server assets
- add a regression test covering the equal-version bundled-vs-downloaded
UI selection path

## Why
- local desktop rebuilds should reflect the latest server and UI code
without requiring users to manually clear cached assets
- packaged updates should keep favoring the freshly bundled frontend
when the cached copy is not actually newer

## Testing
- node --import tsx --test
packages/server/src/ui/__tests__/remote-ui.test.ts
- npm run build:tauri

packages/tauri-app/scripts/prebuild.js

@@ -56,11 +56,7 @@ async function ensureMonacoAssets() {
 function ensureServerBuild() {
   const distPath = path.join(serverRoot, "dist")
   const publicPath = path.join(serverRoot, "public")
-  if (fs.existsSync(distPath) && fs.existsSync(publicPath)) {
-    return
-  }
-
-  console.log("[prebuild] server build missing; running workspace build...")
+  console.log("[prebuild] rebuilding server workspace for desktop packaging...")
   execSync("npm --workspace @neuralnomads/codenomad run build", {
     cwd: workspaceRoot,
     stdio: "inherit",