feat: add graticule toggle control to all Leaflet maps

Adds a bottomleft grid button (MapUtils.addGraticuleControl) to every
map in the app — Meshtastic, MeshCore, Drone, SSTV/ISS, BT Locate,
WebSDR, and Weather Satellite — defaulting to visible. The weather
satellite map's bespoke addStyledGridOverlay() is removed in favour of
the shared implementation. Also updates map-utils.css with button
styles and map-utils.js with the new addGraticuleControl() method.

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

static/js/modes/meshcore.js

@@ -330,6 +330,8 @@ const MeshCore = (function () {
                 Settings.registerMap(_map);
             }).catch(e => console.warn('MeshCore: Settings init failed, using fallback tiles:', e));
         }
+
+        if (typeof MapUtils !== 'undefined') MapUtils.addGraticuleControl(_map);
     }
 
     function _updateMapMarker(node) {