feat(http-frontend): BW bookmark on spectrum, dB floor control, remove FIR taps

Replace the BW slider + FIR taps filter panel with a visual bandwidth
bookmark drawn directly on the spectrum canvas:
- Semi-transparent amber gradient strip spanning dialFreq ± BW/2
- Rounded-top bookmark tab at the top of the strip showing the current BW
- Draggable left/right edge handles (cursor: ew-resize) that adjust bandwidth
  live and send set_bandwidth on mouse-up; range clamped per-mode defaults
- Y-axis now labeled with dB values (floor to ceiling) drawn on canvas
- Configurable floor level via number input below spectrum (default -100 dB)
- Auto button fits floor/range to current noise floor and peak level
- Remove FIR taps selector (internal DSP implementation detail)

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/style.css

@@ -634,6 +634,50 @@ button:focus-visible, input:focus-visible, select:focus-visible {
   white-space: nowrap;
   z-index: 10;
 }
+#spectrum-controls {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  padding: 3px 4px 0;
+  gap: 0.6rem;
+  font-size: 0.78rem;
+  color: var(--text-muted);
+}
+#spectrum-bw-label {
+  font-size: 0.78rem;
+  color: var(--accent-yellow);
+  font-weight: 600;
+  min-width: 5rem;
+}
+#spectrum-level-row {
+  display: flex;
+  align-items: center;
+  gap: 0.4rem;
+}
+#spectrum-floor-label {
+  display: flex;
+  align-items: center;
+  gap: 0.3rem;
+  font-size: 0.75rem;
+  color: var(--text-muted);
+}
+#spectrum-floor-input {
+  width: 3.4rem;
+  padding: 1px 4px;
+  font-size: 0.75rem;
+  border: 1px solid var(--border);
+  border-radius: 4px;
+  background: var(--input-bg);
+  color: var(--text);
+  text-align: right;
+  height: 1.5rem;
+}
+#spectrum-auto-btn {
+  height: 1.5rem;
+  min-height: 0;
+  padding: 0 8px;
+  font-size: 0.73rem;
+}
 #spectrum-hint {
   font-size: 0.68rem;
   color: var(--text-muted);