Show banner when app is built from source

When running the app locally (e.g., `vite dev` or `docker build` without version info), users should see a banner indicating they are on a **source build**.

### Proposal
- Detect missing `APP_VERSION` or mark via env var `IS_SOURCE_BUILD=true`.
- Show banner in the UI: "Running a local development build".
- Should not appear in CI/CD release images.

### Acceptance Criteria
- [ ] Banner appears for local/dev builds with no release version.
- [ ] Banner is absent in Docker images built by CI/CD pipelines.

Start by tracing how `APP_VERSION` and `IS_SOURCE_BUILD` could be supplied during `vite dev` and `docker build`, then locate the UI entry point that can display the banner. Verify the banner appears for local builds without release version information and remains absent from CI/CD release images.

Written by the indexing model from the issue text.