Stale branches arm32 / arm64 / mysqlconnector need the self-contained Dockerfile (or retirement)

Splitting this out of #243 (which removed the now-redundant `build-reactapp` job from `publish.yml` / `test-cd.yml`).

#240 made the Dockerfile self-contained: it now builds the React admin UI in a `ui-build` stage. That change only exists on `master`. Three long-lived branches still carry their own Dockerfile without that stage, plus their own `build-reactapp` CI job that supplies `wwwroot/ui` via artifact.

**They are not broken today** — each branch is internally consistent, and its image still builds with the UI. This is about the drift, not an outage.

| Branch | Last commit | Behind master | Notes |
|---|---|---|---|
| `arm32` | 2024-12-29 | 105 commits | layout matches master (`src/`) |
| `arm64` | 2024-12-29 | 105 commits | layout matches master (`src/`) |
| `mysqlconnector` | 2023-02-20 | 375 commits | pre-`src/` flat layout — projects still at repo root |

Merging master into these is not a small sync. It spans .NET version upgrades, the `src/` directory restructure, and the MongoDB backend work. A conflict dry-run already reports conflicts on `arm32`/`arm64`; `mysqlconnector` predates the directory move entirely, so it's closer to a rebuild than a merge.

Worth deciding per branch:

1. **Still maintained?** If arm32/arm64 images are still published and used, each deserves its own PR: merge master, adopt the `ui-build` Dockerfile stage, drop `build-reactapp`, and verify the resulting arm image actually boots and serves `/ui`.
2. **`mysqlconnector`** — 3.5 years stale and structurally incompatible. If the MySqlConnector work has since landed on master (or is no longer wanted), deleting the branch and its workflow is more honest than carrying it.

Whoever picks this up will want an arm build environment to validate 1 — that verification is the actual cost here, not the merge itself.

Start by comparing the Dockerfile and publish.yml/test-cd.yml on arm32, arm64, and mysqlconnector with master, then determine whether each branch is still maintained. For maintained ARM branches, done means adopting the ui-build stage, removing build-reactapp, and verifying the image boots and serves /ui; for mysqlconnector, decide whether the branch and workflow should be retired.

Written by the indexing model from the issue text.