[fix](trx-frontend): unify signal markers and clamp axis labels

Render the BW and tuned-frequency markers on a shared overlay and keep spectrum axis labels bold and inside their box.

Co-authored-by: OpenAI Codex <codex@openai.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/style.css

@@ -510,14 +510,23 @@ small { color: var(--text-muted); }
   width: 100%;
   margin: 0;
   position: relative;
-  z-index: 4;
 }
 .signal-visual-block {
+  position: relative;
   display: flex;
   flex-direction: column;
   gap: 0;
   margin-bottom: 0.9rem;
 }
+#signal-overlay-canvas {
+  position: absolute;
+  top: 0;
+  left: 0;
+  width: 100%;
+  height: 0;
+  pointer-events: none;
+  z-index: 4;
+}
 #rds-ps-overlay {
   display: none;
   position: absolute;
@@ -1041,6 +1050,7 @@ button:focus-visible, input:focus-visible, select:focus-visible {
   transform: translateX(-50%);
   white-space: nowrap;
   top: 2px;
+  font-weight: 700;
 }
 #spectrum-tooltip {
   display: none;