Add radar overlay on map, fix squawk button and airband status

- Add "Radar" toggle in display controls to overlay radar effect on map
- Radar overlay shows sweep line, range rings, compass rose, center point
- Fix squawk button using addEventListener instead of inline onclick
- Add missing airbandStatus element to fix null error
- Improve squawk modal with click-outside-to-close

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

static/css/adsb_dashboard.css

@@ -440,11 +440,27 @@ body {
 }
 
 #radarMap {
+    position: relative;
     width: 100%;
     height: 100%;
     display: block;
 }
 
+#radarOverlayCanvas {
+    position: absolute;
+    top: 0;
+    left: 0;
+    width: 100%;
+    height: 100%;
+    pointer-events: none;
+    z-index: 500;
+    display: none;
+}
+
+#radarOverlayCanvas.active {
+    display: block;
+}
+
 #radarScope {
     position: absolute;
     top: 0;
@@ -1004,6 +1020,8 @@ body {
     font-family: 'JetBrains Mono', monospace;
     font-size: 10px;
     padding: 0 8px;
+    color: var(--text-muted);
+    white-space: nowrap;
 }
 
 #airbandSquelch {