[feat](trx-frontend): bookmark markers, category colors, text filter

- Move bookmark frequency markers from drawSpectrum() to drawSignalOverlay()
  so dashed lines span both the waterfall and waveform canvases
- Assign distinct palette colors to named categories; uncategorised uses
  --accent-yellow resolved from the live theme at runtime
- Compute WCAG-compliant foreground color (dark/light) per category so label
  text is always legible against the solid background
- Add text search input to the Bookmarks toolbar; filters by name, category,
  and comment client-side without re-fetching from the server

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

@@ -268,6 +268,7 @@
         <select id="bm-category-filter" class="status-input" aria-label="Filter by category">
           <option value="">All categories</option>
         </select>
+        <input type="search" id="bm-text-filter" class="status-input" placeholder="Search bookmarks…" aria-label="Search bookmarks" />
         <button id="bm-add-btn" type="button" class="bm-add-btn" style="display:none;">+ Add Bookmark</button>
       </div>
       <div id="bm-form-wrap" style="display:none;">