Wires the ChannelPreviewPanel (Task 4) into ChannelsScreen: a single
panel instance is rendered per screen and its `channel` prop is swapped
via previewChannelId state rather than remounting per row. The Play
button now reads the server-derived channel.preview.availability
(Task 2) instead of being permanently disabled -- Unavailable stays
disabled with the server's unavailableReason surfaced in the title;
Available and ForcedHlsOnly both enable it, since the panel itself
handles the forced-HLS opt-in and caveat.
Also fixes App.test.tsx's #244 channel fixture, which lacked the now-
required preview field and crashed once ChannelTableRow started
reading it.
timothy
7b21cd98ff