Version Awareness and PR Image Management

## Goal

Improve our Docker build and app runtime experience by:

- Building and tagging images for every PR
- Cleaning up temporary PR images
- Exposing version metadata to the app
- Showing in-app banners for pre-release, source builds, and new versions

## Acceptance Criteria

- [ ] CI/CD builds ephemeral Docker images per PR with predictable naming (`pr-`, `pr--`).
- [ ] ECR is automatically cleaned of stale PR images.
- [ ] The app clearly indicates whether it is a pre-release, release, or local build.
- [ ] Users are informed when a newer version of the app is available.

## Notes

- Lifecycle policies in ECR + GitHub Actions on PR close/merge will handle cleanup.
- Build args / env vars will be injected into Docker builds to expose version metadata.
- Version checks can use either GitHub Releases API or a CI-generated `version.json`.