Moved `handleRepostAction`, `handleMirrorAction`, and `handleEnsurePresenceAction` from `js/app.js` to `js/ui/engagementController.js`.
This follows the AGENTS.md refactor direction for `bitvidApp` to controllers.
- Extracted logic into `EngagementController`.
- Added unit tests for `EngagementController`.
- Wired `EngagementController` into `js/app.js`.
- Preserved `currentVideoPointer` fallback logic in `derivePointerFromDataset`.
