feat: Add BT Locate and GPS modes with IRK auto-detection

New modes:
- BT Locate: SAR Bluetooth device location with GPS-tagged signal trail,
  RSSI-based proximity bands, audio alerts, and IRK auto-extraction from
  paired devices (macOS plist / Linux BlueZ)
- GPS: Real-time position tracking with live map, speed, heading, altitude,
  satellite info, and track recording via gpsd

Bug fixes:
- Fix ABBA deadlock between session lock and aggregator lock in BT Locate
- Fix bleak scan lifecycle tracking in BluetoothScanner (is_scanning property
  now cross-checks backend state)
- Fix map tile persistence when switching modes
- Use 15s max_age window for fresh detections in BT Locate poll loop

Documentation:
- Update README, FEATURES.md, USAGE.md, and GitHub Pages with new modes

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/index.html

@@ -107,6 +107,18 @@
                     <p>Device discovery with tracker detection for AirTags, Tile, Samsung SmartTag, and other Bluetooth devices.</p>
                 </div>
 
+                <div class="feature-card">
+                    <div class="feature-icon">📍</div>
+                    <h3>BT Locate</h3>
+                    <p>SAR Bluetooth device location with GPS-tagged signal trail mapping, IRK-based RPA resolution, and proximity audio alerts.</p>
+                </div>
+
+                <div class="feature-card">
+                    <div class="feature-icon">🛰️</div>
+                    <h3>GPS Tracking</h3>
+                    <p>Real-time GPS position tracking with live map, speed, heading, altitude, satellite info, and track recording.</p>
+                </div>
+
                 <div class="feature-card">
                     <div class="feature-icon">🛡️</div>
                     <h3>TSCM</h3>
@@ -236,6 +248,10 @@
                     <img src="images/ais.png" alt="AIS Vessel Tracking">
                     <span class="screenshot-label">AIS Vessel Tracking</span>
                 </div>
+                <div class="screenshot-item">
+                    <img src="images/bt-locate.png" alt="BT Locate SAR Tracker">
+                    <span class="screenshot-label">BT Locate — SAR Tracker</span>
+                </div>
             </div>
         </div>
     </section>