feat(units): show distances in miles

Everything is computed in kilometres and converted once at display time, in
lib/units — storing miles anywhere would give the same number two sources of
truth and a rounding error that grows with every hop.

The grids capture the unit inside their column definitions, header and formatter
both, so a change is published to them the way the date format already is;
without that a toggle would only appear after a language change or a restart.
The preference is portable, like the other display ones.

frontend/src/components/ChaseNewPanel.tsx

@@ -10,6 +10,7 @@
 // new is being decoded on FT8/FT4/JS8 near here — not that the band is dead.
 import { useEffect, useMemo, useState } from 'react';
 import { Radar, Loader2, X } from 'lucide-react';
+import { formatDistance } from '@/lib/units';
 import { useI18n } from '@/lib/i18n';
 import { markerColour } from '@/lib/spotMarkers';
 import { cn } from '@/lib/utils';
@@ -186,7 +187,7 @@ export function ChaseNewPanel({ onPick, onClose }: Props) {
                   title={[
                     s.country,
                     s.grid,
-                    s.dist_km ? `${s.dist_km} km` : '',
+                    s.dist_km ? formatDistance(s.dist_km) : '',
                     s.freq_hz ? `${(s.freq_hz / 1000).toFixed(1)} kHz` : '',
                   ].filter(Boolean).join(' · ')}
                 >