[feat](trx-frontend-http): add LNA gain control UI and API

Add POST /set_sdr_lna_gain endpoint. Add an LNA Gain input+Set button
to the SDR settings row, styled identically to RF Gain. The control is
hidden until the server reports a sdr_lna_gain_db value (i.e. devices
that expose an LNA gain element). AGC disables it alongside RF Gain.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

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

@@ -268,6 +268,13 @@
                   </label>
                   <button id="sdr-gain-set" type="button" class="wfm-inline-btn">Set</button>
                 </div>
+                <div class="wfm-gain-group" id="sdr-lna-gain-controls">
+                  <label class="wfm-control">
+                    <span class="wfm-control-label">LNA Gain</span>
+                    <input id="sdr-lna-gain-db" class="status-input" type="number" min="0" max="60" step="1" inputmode="decimal">
+                  </label>
+                  <button id="sdr-lna-gain-set" type="button" class="wfm-inline-btn">Set</button>
+                </div>
               </div>
             </div>
             <div class="full-row label-below-row" id="vchan-row">