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/sstv.js

@@ -241,6 +241,8 @@ const SSTV = (function() {
             }
         }
 
+        if (typeof MapUtils !== 'undefined') MapUtils.addGraticuleControl(issMap);
+
         // Create ISS icon
         const issIcon = L.divIcon({
             className: 'sstv-iss-marker',