v1.2.0 — Beta channel autoupdate
`Application.AutoUpdate` gains a new `AllowPrerelease bool` field. Set it to true to receive GitHub releases marked as "prerelease" through the autoupdate flow.
```go
menuet.App().AutoUpdate.Version = "v1.2.0"
menuet.App().AutoUpdate.Repo = "caseymrm/myapp"
menuet.App().AutoUpdate.AllowPrerelease = true // opt-in
```
Default is false (stable channel) — same behavior as before, just now we explicitly filter prereleases out instead of accidentally treating them as stable.
Apps already on a prerelease tag are not downgraded by the stable channel — switching channels is a user action.
No other API changes. Tests cover both modes (5 new table-driven cases).