Show in-app banner when a new version is available

Users should be notified if they are running an outdated version of the app.

### Proposal
- On app load (and periodically), fetch version info:
- Option A: Use GitHub Releases API.
- Option B: Use a hosted `version.json` file updated by CI/CD.
- Compare `APP_VERSION` with the latest release (or latest-snapshot for prereleases).
- Show banner: "A new version (vX.Y.Z) is available — please refresh or upgrade."

### Acceptance Criteria
- [ ] App can check for latest available version.
- [ ] Banner appears when a newer version exists.
- [ ] Clicking banner refreshes or provides upgrade instructions.
- [ ] No banner appears if app is up to date.