[feat](trx-frontend-http): display RDS PS name overlay on overview strip

When a WFM/SDR spectrum stream carries RDS data, show the Program
Service name (8-char station name) centered on the visible waveform
area in DSEG14 monospace — the same font as the frequency display.

- Add #rds-ps-overlay div inside .overview-strip (pointer-events: none,
  z-index: 2, absolutely positioned at the center of the visible canvas)
- updateRdsPsOverlay(rds) shows/hides and updates text on every spectrum
  frame; trims trailing spaces common in RDS PS strings
- Overlay cleared on spectrum disconnect / null frame
- text-shadow uses CSS color-mix against --bg for legibility across all
  style/theme combinations

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

@@ -48,6 +48,7 @@
     </div>
     <div class="overview-strip">
       <canvas id="overview-canvas" aria-hidden="true"></canvas>
+      <div id="rds-ps-overlay" aria-live="polite" aria-label="RDS station name"></div>
     </div>
     <!-- Auth gate (hidden by default, shown if auth is required) -->
     <div id="auth-gate" style="display:none; max-width: 30rem; margin: 0 auto 0.9rem; padding: 1.25rem 0 1.5rem; text-align: center;">