[feat](trx-frontend-http): add keyboard hotkeys for radio controls

Map R=retune, B=previous state, [/]=bandwidth ±10kHz, arrows=tune/center,
M=mode picker, Z=mono/stereo, N=noise blanker, Q=squelch toggle.
Document all shortcuts in the F1 help overlay.

https://claude.ai/code/session_01WDC889uQGW9XoPQqSZ6bVt
Signed-off-by: Claude <noreply@anthropic.com>

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

@@ -1165,7 +1165,18 @@
       <div class="shortcut-overlay-title">Keyboard Shortcuts</div>
       <table class="shortcut-table">
         <tbody>
+          <tr><td class="shortcut-key"><kbd>R</kbd></td><td>Retune current frequency</td></tr>
+          <tr><td class="shortcut-key"><kbd>B</kbd></td><td>Jump to previous freq/bw/mode</td></tr>
+          <tr><td class="shortcut-key"><kbd>[</kbd> / <kbd>]</kbd></td><td>Narrow / widen bandwidth (±10 kHz)</td></tr>
+          <tr><td class="shortcut-key"><kbd>←</kbd> / <kbd>→</kbd></td><td>Tune by current step</td></tr>
+          <tr><td class="shortcut-key"><kbd>↑</kbd> / <kbd>↓</kbd></td><td>Shift center frequency</td></tr>
+          <tr><td class="shortcut-key"><kbd>M</kbd></td><td>Open mode picker</td></tr>
+          <tr><td class="shortcut-key"><kbd>Z</kbd></td><td>Toggle mono / stereo (WFM)</td></tr>
+          <tr><td class="shortcut-key"><kbd>N</kbd></td><td>Toggle noise blanker</td></tr>
+          <tr><td class="shortcut-key"><kbd>Q</kbd></td><td>Toggle squelch (off / auto)</td></tr>
           <tr><td class="shortcut-key"><kbd>S</kbd></td><td>Spectrum screenshot</td></tr>
+          <tr><td class="shortcut-key"><kbd>+</kbd> / <kbd>-</kbd></td><td>Zoom spectrum in / out</td></tr>
+          <tr><td class="shortcut-key"><kbd>0</kbd></td><td>Reset spectrum zoom</td></tr>
           <tr><td class="shortcut-key"><kbd>F1</kbd></td><td>Toggle this help</td></tr>
           <tr><td class="shortcut-key"><kbd>Esc</kbd></td><td>Close overlay / exit fullscreen</td></tr>
         </tbody>