Fix multiple UI bugs and improve error handling

Issues fixed:
- #113: Display RTL-SDR serial numbers in device selector
- #112: Kill all processes now stops Bluetooth scans
- #111: BLE device list no longer overflows container bounds
- #109: WiFi scanner panels maintain minimum width (no more "imploding")
- #108: Radar device hover no longer causes violent shaking
- #106: "Use GPS" button now uses gpsd for USB GPS devices
- #105: Meter trend text no longer overlaps adjacent columns
- #104: dump1090 errors now provide specific troubleshooting guidance

Changes:
- app.py: Add Bluetooth cleanup to /killall endpoint
- routes/adsb.py: Parse dump1090 stderr for specific error messages
- templates/index.html: Show SDR serial numbers in device dropdown
- static/css/index.css: Fix WiFi/BT panel layouts with proper min-width
- static/css/components/signal-cards.css: Fix meter grid overflow
- static/css/components/proximity-viz.css: Fix radar hover transform
- static/css/settings.css: Add GPS detection spinner
- static/js/components/proximity-radar.js: Add invisible hit areas
- static/js/core/settings-manager.js: Use gpsd before browser geolocation

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

static/css/settings.css

@@ -326,6 +326,23 @@
     cursor: not-allowed;
 }
 
+/* GPS Detection Spinner */
+.detecting-spinner {
+    display: inline-block;
+    width: 12px;
+    height: 12px;
+    border: 2px solid currentColor;
+    border-top-color: transparent;
+    border-radius: 50%;
+    animation: detecting-spin 0.8s linear infinite;
+    vertical-align: middle;
+    margin-right: 6px;
+}
+
+@keyframes detecting-spin {
+    to { transform: rotate(360deg); }
+}
+
 /* About Section */
 .about-info {
     font-size: 13px;