[feat](trx-frontend-http): add RDS debug panel in Plugins tab

Add an RDS sub-tab to the Plugins panel showing PI code, PS name, PTY
number and name, decoder status, and a raw JSON dump of the latest RDS
data received via SSE. Also list the RDS decoder in the Overview tab.

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

@@ -236,6 +236,7 @@
         <button class="sub-tab" data-subtab="cw">CW</button>
         <button class="sub-tab" data-subtab="ft8">FT8</button>
         <button class="sub-tab" data-subtab="wspr">WSPR</button>
+        <button class="sub-tab" data-subtab="rds">RDS</button>
         <button class="sub-tab" data-subtab="map">Map</button>
       </div>
       <div id="subtab-overview" class="sub-tab-panel">
@@ -263,6 +264,23 @@
             Decodes WSPR messages from RX audio (DIG/USB only, toggle required).
           </div>
         </div>
+        <div class="plugin-item">
+          <strong>RDS Decoder</strong>
+          <div style="color:var(--text-muted); font-size:0.85rem; margin-top:0.2rem;">
+            Decodes Radio Data System (RDS) metadata from WFM broadcasts (57 kHz subcarrier).
+          </div>
+        </div>
+      </div>
+      <div id="subtab-rds" class="sub-tab-panel" style="display:none;">
+        <div class="rds-grid">
+          <div class="rds-field"><span class="rds-label">Status</span><span id="rds-status" class="rds-value rds-no-signal">No signal</span></div>
+          <div class="rds-field"><span class="rds-label">PI</span><span id="rds-pi" class="rds-value">--</span></div>
+          <div class="rds-field"><span class="rds-label">PS</span><span id="rds-ps" class="rds-value rds-ps">--</span></div>
+          <div class="rds-field"><span class="rds-label">PTY</span><span id="rds-pty" class="rds-value">--</span></div>
+          <div class="rds-field"><span class="rds-label">PTY Name</span><span id="rds-pty-name" class="rds-value">--</span></div>
+        </div>
+        <div class="rds-raw-label">Raw JSON</div>
+        <pre id="rds-raw" class="rds-raw">--</pre>
       </div>
       <div id="subtab-map" class="sub-tab-panel" style="display:none;">
         <div class="map-controls">