fix: declare geoFilterLayer in map.js strict mode scope

In strict mode, assigning to an undeclared variable throws a
ReferenceError which was silently caught, hiding the overlay entirely.

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

public/map.js

@@ -12,6 +12,7 @@
   let filters = { repeater: true, companion: true, room: true, sensor: true, observer: true, lastHeard: '30d', neighbors: false, clusters: false, hashLabels: localStorage.getItem('meshcore-map-hash-labels') !== 'false', statusFilter: localStorage.getItem('meshcore-map-status-filter') || 'all' };
   let wsHandler = null;
   let heatLayer = null;
+  let geoFilterLayer = null;
   let userHasMoved = false;
   let controlsCollapsed = false;