Extracted video reaction logic (subscription, optimistic updates, and publishing) from `js/app.js` into a dedicated `ReactionController`.
* Created `js/ui/reactionController.js`.
* Refactored `js/app.js` to usage `ReactionController` and removed legacy methods.
* Added unit tests in `tests/ui/reactionController.test.mjs`.
* Ensured reaction subscription updates on user login/logout.
This follows the `AGENTS.md` guideline to move UI logic to controllers.
