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/server/src/ui/remote-ui.ts

@@ -250,7 +250,7 @@ async function pickBestLocalUi(args: {
       uiStaticDir: currentResolved,
       source: "downloaded",
       uiVersion: await readUiVersion(currentResolved),
-      priority: 2,
+      priority: 1,
     })
   }
 
@@ -260,7 +260,7 @@ async function pickBestLocalUi(args: {
       uiStaticDir: bundledResolved,
       source: "bundled",
       uiVersion: await readUiVersion(bundledResolved),
-      priority: 1,
+      priority: 2,
     })
   }