[feat](trx-frontend-http): add style picker with Nord, Monokai, Contrast themes

Add a style picker dropdown to the tab bar (right of rig picker) with
four styles — Original, Nord, Monokai, Contrast — each with full
light/dark variants.

CSS: define data-style attribute overrides for all CSS custom properties
(bg, card-bg, borders, text, accents, jog, audio level, filter, spectrum
background) for each of the three new styles × two themes (6 new blocks).

JS: introduce CANVAS_PALETTE lookup table covering spectrum/waveform/
waterfall colors for all style×theme combinations. Add currentStyle(),
canvasPalette(), setStyle() helpers. Persist selection to localStorage.

Replace all isLight ternaries in drawing code with palette lookups:
- drawOverviewWaterfall, drawOverviewSignalHistory, waterfallColor
  signatures changed from isLight flag to pal object
- drawSpectrum uses canvasPalette() for grid lines, labels, fill, line
- spectrumBgColor() now delegates to canvasPalette().bg

Theme toggle also triggers a spectrum redraw so canvas colors update
immediately when switching light/dark.

Also fix light-theme spectrum rendering broken since canvas drawing used
hardcoded dark-only colors (white grid lines invisible on light bg).

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

@@ -22,6 +22,14 @@
         <div class="header-rig-switch">
           <select id="header-rig-switch-select" aria-label="Select active rig"></select>
         </div>
+        <div class="header-style-pick">
+          <select id="header-style-pick-select" aria-label="Select UI style">
+            <option value="original">Original</option>
+            <option value="nord">Nord</option>
+            <option value="monokai">Monokai</option>
+            <option value="contrast">Contrast</option>
+          </select>
+        </div>
         <button id="theme-toggle" class="header-bar-btn" type="button" aria-label="Toggle dark or light theme">Light</button>
         <button id="header-auth-btn" class="header-bar-btn" type="button" style="display:none;" aria-label="Login or Logout">Login</button>
       </div>