Channels are recall-of-saved-config — a client of the radio core, not part
of DSP/network. Move ApplyChannel/CheckChannelActive + active-channel state
(FActiveChannelIdx, FPreChannelPower) out of MainForm into TChannelController,
which orchestrates controller commands and emits OnActiveChanged for the
Channel-button render. Removes the last UI-held state blocking headless mode.
Also completes TRadioController.SetDrive (was a skeleton): clamp + CalcDriveByte
+ SetDriveLevel + PushNetworkState + Changed(rfDrive), with an rfDrive render
case; routes slider/web/channel drive through it. Adds SetCurrentBandIdx for the
lightweight HF band switch.
Behavior note: channel recall now applies the mode-default filter via SetMode
(was: kept current filter — a latent quirk that left FM on an SSB bandwidth).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
