perf(beacons): coverage map uses Leaflet canvas renderer, no sticky tooltips

The 'show estimated current coverage' toggle pushes 1-8k grid cells
to the browser at 5.76 GHz+ and previously rendered each as its own
SVG <path> via Leaflet's default vector renderer. That meant 1-8k
DOM nodes plus per-element layout/paint on every pan/zoom — enough
to lock up tabs on mid-range hardware.

Two changes:

1. All coverage rects share a single L.canvas({padding: 0.2})
   renderer. Browser sees one <canvas> element drawn in one pass
   per redraw instead of thousands of <path> nodes. Hit-testing for
   tooltips still works through Leaflet's canvas-renderer mouse
   events.

2. Drop `sticky: true` from the tooltip binding. Sticky tooltips
   reposition to follow the cursor on every mousemove, which scales
   O(N) with cell count — across thousands of rects that becomes the
   dominant cost as the cursor moves. Without sticky, the tooltip
   anchors to the cell's centre on hover (same info, cheaper).

assets/js/beacon_map_hook.ts

@@ -61,6 +61,23 @@ export const BeaconMap = {
     // event, which the server only sends when the user flips the
     // toggle on. Keeping this off the mount path avoids parsing tens
     // of thousands of cells into Leaflet rects on every page load.
+    //
+    // Coverage rects all share a single L.canvas() renderer instead
+    // of Leaflet's default SVG renderer. With SVG, each cell becomes
+    // its own <path> element — at 5.76 GHz+ a typical estimate is
+    // ~3-8k cells, which means ~3-8k DOM nodes plus per-element layout
+    // / paint work on every pan/zoom. Canvas collapses that to a
+    // single <canvas> element, drawn in one pass per redraw.
+    //
+    // Hit-testing for tooltips still works through Leaflet's canvas
+    // renderer (it walks the layer tree at mousemove time), so the
+    // per-cell tooltip is preserved. We do drop `sticky: true`
+    // though: that flag re-projects the tooltip to follow the cursor
+    // pixel-by-pixel, which against thousands of rects compounds
+    // every mousemove with N geometry checks. Without sticky, the
+    // tooltip anchors to the cell's centre — same information,
+    // multiple-orders-of-magnitude cheaper.
+    const coverageRenderer = L.canvas({padding: 0.2})
     let cellLayer: L.LayerGroup | null = null
     let cellBounds: [number, number][] = []
 
@@ -77,6 +94,7 @@ export const BeaconMap = {
         const sw: [number, number] = [c.lat - half, c.lon - half]
         const ne: [number, number] = [c.lat + half, c.lon + half]
         const rect = L.rectangle([sw, ne], {
+          renderer: coverageRenderer,
           color: c.color,
           weight: 0,
           fillColor: c.color,
@@ -84,8 +102,7 @@ export const BeaconMap = {
           interactive: true
         })
         rect.bindTooltip(
-          `${c.label}<br>${c.rx_dbm} dBm<br>${formatDistanceKm(c.distance_km)} · score ${c.score}`,
-          {sticky: true}
+          `${c.label}<br>${c.rx_dbm} dBm<br>${formatDistanceKm(c.distance_km)} · score ${c.score}`
         )
         rect.addTo(cellLayer)
         cellBounds.push(sw, ne)