fix(map): Zoom DX clamps its frame to Mercator's edge

The great-circle arc to a polar entity (Franz Josef Land) peaks near 88°N;
fitting the raw arc points framed a band of tile-less white above 85°, where
Web Mercator ends. The camera's bounds are clamped to ±85° — the line still
draws to wherever it goes, only the framing stays where there is a map.

frontend/src/components/MainMap.tsx

@@ -370,8 +370,14 @@ export function WorldMap({ fromGrid, toGrid, fromLabel, toLabel, beamAzimuths, b
 
     if (autoZoom) {
       if (from && to && arcPts) {
-        const bounds = L.latLngBounds([[from.lat, from.lon], [to.lat, to.lon]]);
-        arcPts.forEach((p) => bounds.extend(p as L.LatLngExpression));
+        // Latitudes clamped to Mercator's edge (±85°): the arc to a polar
+        // entity (Franz Josef Land) peaks near 88°N, and fitting the raw
+        // points framed a band of tile-less white above the top of the world.
+        // The line itself still draws to wherever it goes — only the CAMERA
+        // stays where there is a map to show.
+        const clamp = (lat: number) => Math.max(-85, Math.min(85, lat));
+        const bounds = L.latLngBounds([[clamp(from.lat), from.lon], [clamp(to.lat), to.lon]]);
+        arcPts.forEach((p) => bounds.extend([clamp(p[0]), p[1]] as L.LatLngExpression));
         wm.fitBounds(bounds, { padding: [30, 30], maxZoom: 6 });
       } else if (to) {
         wm.setView([to.lat, to.lon], 3);