[feat](trx-frontend-http): overlay bookmarks on spectrum; enforce one per freq

Draw bookmark frequency markers on the spectrum canvas: amber vertical
line + ribbon shape (rectangle with V-notch) at each bookmark in view.
Below the freq axis, show a #spectrum-bookmark-axis row of clickable
amber ribbon labels (clip-path bookmark shape); clicking tunes the rig.
Labels auto-appear / collapse as bookmarks scroll in and out of view.

Server: reject POST/PUT with 409 Conflict when another bookmark already
exists at the requested freq_hz (BookmarkStore::freq_taken helper).

Client: bmFetch() triggers a spectrum redraw so markers appear
immediately on load without requiring a tab visit first.

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

@@ -82,6 +82,7 @@
           <canvas id="spectrum-canvas"></canvas>
           <div id="spectrum-tooltip"></div>
           <div id="spectrum-freq-axis"></div>
+          <div id="spectrum-bookmark-axis"></div>
         </div>
         <div id="spectrum-controls">
           <div id="spectrum-bw-row">