[feat](trx-frontend-http): add decoder toggle/clear UI and endpoints

Add POST endpoints for toggle_aprs_decode, toggle_cw_decode,
clear_aprs_decode, and clear_cw_decode. Add toggle buttons in APRS
and CW tabs. Render decoder enabled state from SSE updates. Clear
button now also resets server-side decoder state.

Co-Authored-By: Claude Opus 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

@@ -145,6 +145,7 @@
       </div>
       <div id="subtab-aprs" class="sub-tab-panel" style="display:none;">
         <div class="aprs-controls">
+          <button id="aprs-decode-toggle-btn" type="button">Enable APRS</button>
           <button id="aprs-clear-btn" type="button">Clear</button>
           <small id="aprs-status" style="color:var(--text-muted);">Waiting for server decode</small>
         </div>
@@ -152,6 +153,7 @@
       </div>
       <div id="subtab-cw" class="sub-tab-panel" style="display:none;">
         <div class="cw-controls">
+          <button id="cw-decode-toggle-btn" type="button">Enable CW</button>
           <button id="cw-clear-btn" type="button">Clear</button>
           <small id="cw-status" style="color:var(--text-muted);">Waiting for server decode</small>
           <div id="cw-signal-indicator" class="cw-signal-off"></div>