[feat](trx-frontend-http): add R hotkey to round frequency up to next step

Remap retune from R to T, and add a new R hotkey that rounds the
current frequency forward to the next jog-step boundary. Both the
new hotkey and the remapped retune are documented in the F1 help
overlay.

https://claude.ai/code/session_017neG2jL9uXFSRpmhyS1EqG
Signed-off-by: Claude <noreply@anthropic.com>

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

@@ -1165,7 +1165,8 @@
       <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>R</kbd></td><td>Round frequency up to next step</td></tr>
+          <tr><td class="shortcut-key"><kbd>T</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>