feat: Add Leaflet map for contact locations

- Replace Google Maps with Leaflet + OpenStreetMap (free, no API key)
- Add Map button in main menu to show all contacts with GPS
- Add Map button on message bubbles (next to Reply) for senders with GPS
- Contact Management Map buttons now open modal instead of new tab
- Lazy map initialization with proper Bootstrap modal handling

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

app/static/css/style.css

@@ -698,3 +698,21 @@ main {
     opacity: 0.6;
     cursor: not-allowed;
 }
+
+/* =============================================================================
+   Leaflet Map Modal
+   ============================================================================= */
+
+#mapModal .modal-body {
+    min-height: 400px;
+}
+
+#leafletMap {
+    z-index: 1;
+}
+
+/* Fix Leaflet controls z-index in Bootstrap modal */
+.leaflet-top,
+.leaflet-bottom {
+    z-index: 1000;
+}