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/websdr.js

@@ -343,6 +343,7 @@ async function initWebsdrLeaflet(mapEl) {
         }
     }
 
+    if (typeof MapUtils !== 'undefined') MapUtils.addGraticuleControl(websdrMap);
     mapEl.style.background = '#1a1d29';
     return true;
 }