[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-client/trx-frontend/trx-frontend-http/assets/web/index.html

@@ -232,13 +232,6 @@
                       <option value="high">High</option>
                     </select>
                   </label>
-                  <div class="wfm-gain-group" id="sdr-gain-controls">
-                    <label class="wfm-control">
-                      <span class="wfm-control-label">RF Gain</span>
-                      <input id="sdr-gain-db" class="status-input" type="number" min="0" max="60" step="1" inputmode="decimal">
-                    </label>
-                    <button id="sdr-gain-set" type="button" class="wfm-inline-btn">Set</button>
-                  </div>
                   <label class="wfm-control wfm-st-flag-wrap" aria-label="Stereo pilot status">
                     <span class="wfm-control-label">Pilot</span>
                     <span id="wfm-st-flag" class="wfm-st-flag wfm-st-flag-mono">MO</span>
@@ -259,6 +252,22 @@
               <div class="label"><span>VFO</span></div>
               <div class="vfo-picker" id="vfo-picker"></div>
             </div>
+            <div class="full-row label-below-row" id="sdr-settings-row" style="display:none;">
+              <div class="label"><span>SDR settings</span></div>
+              <div class="inline" style="gap: 0.6rem; flex-wrap: wrap; align-items: center;">
+                <label class="vol-label" id="sdr-agc-wrap">
+                  <span style="margin-right:0.3rem;">Hardware AGC</span>
+                  <input type="checkbox" id="sdr-agc-enabled">
+                </label>
+                <div class="wfm-gain-group" id="sdr-gain-controls">
+                  <label class="wfm-control">
+                    <span class="wfm-control-label">RF Gain</span>
+                    <input id="sdr-gain-db" class="status-input" type="number" min="0" max="60" step="1" inputmode="decimal">
+                  </label>
+                  <button id="sdr-gain-set" type="button" class="wfm-inline-btn">Set</button>
+                </div>
+              </div>
+            </div>
             <div class="full-row label-below-row" id="vchan-row">
               <div class="label"><span>Channels / Scheduler</span></div>
               <div class="channel-scheduler-controls">