git: stop tracking bundled build artifacts

These files are generated at build time and shouldn't be committed:
- dist/control-ui assets (JS/CSS bundles)
- src/canvas-host/a2ui bundle files

This removes ~100MB+ of bloat from git history by no longer tracking
repeatedly regenerated bundle files. Add to .gitignore to prevent
accidental re-addition.

Co-Authored-By: Claude <noreply@anthropic.com>

.gitignore

@@ -40,6 +40,9 @@ apps/ios/*.xcfilelist
 
 # Vendor build artifacts
 vendor/a2ui/renderers/lit/dist/
+src/canvas-host/a2ui/*.bundle.js
+src/canvas-host/a2ui/*.map
+src/canvas-host/a2ui/index.html
 .bundle.hash
 
 # fastlane (iOS)