Review findings on #135949 (Greptile, @unsupportedpastels):
- An older runtime (v0.21.6) answers checks but rejects --set-channel, so the
selector failed on every click. source_check now marks its reply
`channelSelectable`; Desktop shows the row only when the install says so.
- The confirmed choice painted only after the forced check returned. The row
now shows it at once, and a failure re-reads the install instead of guessing.
- A checkout following a custom branch rendered as "Every commit" (main). The
row now appears only for stable, or for main itself.
a7b2ba02e3