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/js/components/proximity-radar.js

@@ -207,9 +207,14 @@ const ProximityRadar = (function() {
             const pulseClass = isNew ? 'radar-dot-pulse' : '';
             const isSelected = selectedDeviceKey && device.device_key === selectedDeviceKey;
 
+            // Hit area size (prevents hover flicker when scaling)
+            const hitAreaSize = Math.max(dotSize * 2, 15);
+
             return `
                 <g class="radar-device ${pulseClass}${isSelected ? ' selected' : ''}" data-device-key="${escapeAttr(device.device_key)}"
                    transform="translate(${x}, ${y})" style="cursor: pointer;">
+                    <!-- Invisible hit area to prevent hover flicker -->
+                    <circle class="radar-device-hitarea" r="${hitAreaSize}" fill="transparent" />
                     ${isSelected ? `<circle r="${dotSize + 8}" fill="none" stroke="#00d4ff" stroke-width="2" stroke-opacity="0.8">
                         <animate attributeName="r" values="${dotSize + 6};${dotSize + 10};${dotSize + 6}" dur="1.5s" repeatCount="indefinite"/>
                         <animate attributeName="stroke-opacity" values="0.8;0.4;0.8" dur="1.5s" repeatCount="indefinite"/>