Clean up and improve non-gitea js error filter (#37148)

1. Filter out errors that contain `chrome-extension://` etc protocols
2. Extract filtering into its own function and test it
3. Fix the `window.config.assetUrlPrefix` mock, guaranteed to end with
`/assets`
4. Remove useless `??` and `?.` for properties that always exist

Co-authored-by: Claude (Opus 4.6) <noreply@anthropic.com>

web_src/js/vitest.setup.ts

@@ -13,7 +13,7 @@ await import('./globals.ts');
 window.config = {
   appUrl: 'http://localhost:3000/',
   appSubUrl: '',
-  assetUrlPrefix: '',
+  assetUrlPrefix: '/assets',
   sharedWorkerUri: '',
   runModeIsProd: true,
   customEmojis: {},