[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/plugins/bookmarks.js

@@ -46,6 +46,7 @@ async function bmFetch(categoryFilter) {
   bmSyncAccess();
   bmRender(bmList);
   bmRefreshCategoryFilter(categoryFilter);
+  if (typeof scheduleSpectrumDraw === "function") scheduleSpectrumDraw();
 }
 
 async function bmRefreshCategoryFilter(keepValue) {
@@ -197,6 +198,9 @@ async function bmSave(e) {
     }
     if (!resp.ok) {
       const text = await resp.text();
+      if (resp.status === 409) {
+        throw new Error("A bookmark for that frequency already exists.");
+      }
       throw new Error(text || "HTTP " + resp.status);
     }
     bmCloseForm();
@@ -294,4 +298,7 @@ async function bmApply(bm) {
       await bmDelete(delBtn.dataset.bmId);
     }
   });
+
+  // Pre-load bookmarks so spectrum markers are visible immediately.
+  bmFetch("");
 })();