[feat](trx-backend-soapysdr): add hardware AGC toggle and SDR settings UI row

Add hardware AGC on/off control for SoapySDR backend, wired through the
full stack from RigCommand to the web UI:

- RigCommand::SetSdrAgc(bool) + ClientCommand::SetSdrAgc in protocol
- set_sdr_agc() on RigCat trait (not-supported default)
- SoapySdrRig: agc_enabled field, set_sdr_agc() via pipeline agc_cmd,
  sdr_agc_enabled in filter_state(); removes the "not yet implemented"
  warning — gain_mode="auto" now properly enables hardware AGC via
  SoapySDR set_gain_mode()
- IqSource::set_gain_mode() trait method; RealIqSource implements it
- SdrPipeline: agc_cmd channel, read loop applies it each iteration
- POST /set_sdr_agc endpoint in trx-frontend-http
- New "SDR settings" full-row in index.html with Hardware AGC checkbox
  and RF Gain (moved out of WFM controls); row hidden when
  show_sdr_gain_control is false
- app.js: AGC checkbox handler, disables RF gain input while AGC is on,
  syncs checkbox state from filter.sdr_agc_enabled

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-protocol/src/mapping.rs

@@ -57,6 +57,7 @@ pub fn client_command_to_rig(cmd: ClientCommand) -> RigCommand {
         ClientCommand::SetBandwidth { bandwidth_hz } => RigCommand::SetBandwidth(bandwidth_hz),
         ClientCommand::SetFirTaps { taps } => RigCommand::SetFirTaps(taps),
         ClientCommand::SetSdrGain { gain_db } => RigCommand::SetSdrGain(gain_db),
+        ClientCommand::SetSdrAgc { enabled } => RigCommand::SetSdrAgc(enabled),
         ClientCommand::SetSdrSquelch {
             enabled,
             threshold_db,
@@ -119,6 +120,7 @@ pub fn rig_command_to_client(cmd: RigCommand) -> ClientCommand {
         RigCommand::SetBandwidth(bandwidth_hz) => ClientCommand::SetBandwidth { bandwidth_hz },
         RigCommand::SetFirTaps(taps) => ClientCommand::SetFirTaps { taps },
         RigCommand::SetSdrGain(gain_db) => ClientCommand::SetSdrGain { gain_db },
+        RigCommand::SetSdrAgc(enabled) => ClientCommand::SetSdrAgc { enabled },
         RigCommand::SetSdrSquelch {
             enabled,
             threshold_db,

src/trx-protocol/src/types.rs

@@ -46,6 +46,7 @@ pub enum ClientCommand {
     SetBandwidth { bandwidth_hz: u32 },
     SetFirTaps { taps: u32 },
     SetSdrGain { gain_db: f64 },
+    SetSdrAgc { enabled: bool },
     SetSdrSquelch { enabled: bool, threshold_db: f64 },
     SetWfmDeemphasis { deemphasis_us: u32 },
     SetWfmStereo { enabled: bool },