fix(app-bridge-react): stabilize SaveBar and Modal ref callbacks

[HamedFarazi]

Summary

Fixes #565

SaveBar and Modal were creating a new inline ref callback on every render.
React treats changing callback refs as detach/attach operations, causing the
ref callback to run repeatedly and trigger internal state updates.

Changes

• Replaced inline ref callbacks with memoized useCallback refs
• Preserved existing forwardedRef behavior for:
  • callback refs
  • object refs
  • unmount cleanup

Validation

• ✅ pnpm lint
• ✅ pnpm test
• ✅ TypeScript diagnostics clean

Only modified:

• packages/app-bridge-react/src/components/SaveBar.tsx
• packages/app-bridge-react/src/components/Modal.tsx

[henrytao-me]

Thanks @HamedFarazi for sending the PR 👍