feat: Estimate contact location from RSSI + last-hop repeater

For contacts without GPS, estimate their position by:
1. Finding the first-hop repeater from their routing path
2. Estimating distance from RSSI using log-distance path loss model
3. Placing them at a deterministic offset from the repeater

Uses LoRa outdoor path loss exponent (n=3.0) with reference RSSI
-30dBm at 1m. Distance clamped to 10m-50km range. Bearing derived
from contact public key hash for stable positioning.

lib/services/map_marker_service.dart

@@ -41,10 +41,12 @@ class MapMarkerService {
     Function(Contact)? onTap,
     double mapRotation = 0,
     Position? userPosition,
+    Map<String, LatLng>? estimatedLocations,
   }) {
     return contacts
         .map((contact) {
-          final location = contact.displayLocation;
+          final location = contact.displayLocation ??
+              estimatedLocations?[contact.publicKeyHex];
           if (location == null) return null;
 
           return Marker(