Platform builds are only exercised at release time

Release builds only ever run on a tag, so the first time a change is compiled for Windows, macOS, iOS or Linux is when a release is being cut. A platform break is discovered at the worst possible moment, and `ci.yml` (ubuntu, analyze + test) does not compile any platform.

- [ ] Scheduled workflow running `flutter build` per platform on the release matrix
- [ ] `workflow_dispatch` so it can be triggered before cutting a tag
- [ ] Decide whether it needs the Firebase secrets (it does for a real Android build) or whether the stub is enough to prove compilation
- [ ] Keep it off the PR path — full platform builds are too slow to gate merges

Depends on #28, so the four setup blocks are one composite action before a fifth is added.