[feat](trx-frontend-http): improve mobile UX and fix S0/S9 label offset

- Add viewport meta tag so mobile browsers use device width instead of
  the default 980px desktop viewport
- Add 520px breakpoint: remove controls-tray forced min-width (was
  52–58rem, causing horizontal scroll on phones), stack controls-row
  into a single column with jog wheel first (order: -1)
- Scale frequency DSEG14 display with clamp() on narrow screens
- Make volume sliders responsive width with larger 20px thumb
- Raise spectrum Set/Auto button and input heights to 2.2rem (overrides
  the min-height: 0 that blocked the existing 760px touch-size rule)
- Add overflow-x: auto + flex-shrink: 0 to sub-tab-bar so all six
  plugin tabs are reachable on small screens without clipping
- Swap spectrum hint text based on input device: mouse/keyboard hint
  hidden on touch devices, touch-specific hint shown instead
- Offset S0/S9/S9+ signal history labels 6px below their grid lines

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/app.js

@@ -654,7 +654,7 @@ function drawOverviewSignalHistory(ctx, w, h, isLight) {
     ctx.moveTo(0, y);
     ctx.lineTo(w, y);
     ctx.stroke();
-    ctx.fillText(marker.label, w - 6, Math.max(8, Math.min(h - 8, y)));
+    ctx.fillText(marker.label, w - 6, Math.max(8, Math.min(h - 8, y + 6)));
   }
 
   ctx.beginPath();