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/lib/units.ts

@@ -0,0 +1,49 @@
+// Distance units.
+//
+// Everything is COMPUTED in kilometres — the great-circle maths, the backend's
+// distance_km on a spot, the map's path lengths — and converted once, here, at
+// display time. Storing miles anywhere would mean two sources of truth for the
+// same number and a rounding error that grows with every hop.
+//
+// The preference is portable (see lib/uiPref): an operator who works in miles
+// works in miles on every machine they copy their folder to.
+import { writeUiPref } from '@/lib/uiPref';
+
+export const KEY_MILES = 'opslog.distanceMiles';
+const KM_PER_MILE = 1.609344; // statute miles, the ones a US licence is used in
+
+export function useMiles(): boolean {
+  try { return localStorage.getItem(KEY_MILES) === '1'; } catch { return false; }
+}
+
+export function setUseMiles(on: boolean): void {
+  writeUiPref(KEY_MILES, on ? '1' : '0');
+  listeners.forEach((l) => l());
+}
+
+// subscribeDistanceUnit notifies on a change. The grids capture the unit inside
+// their column definitions (header text and formatter both), so without this a
+// toggle would only show up on the next language change or restart.
+const listeners = new Set<() => void>();
+export function subscribeDistanceUnit(fn: () => void): () => void {
+  listeners.add(fn);
+  return () => { listeners.delete(fn); };
+}
+
+// distanceValue converts a distance in km to the operator's unit, rounded to a
+// whole unit — the precision the inputs actually justify (a 4-character grid is
+// a square tens of kilometres wide).
+export function distanceValue(km: number): number {
+  if (!isFinite(km)) return 0;
+  return Math.round(useMiles() ? km / KM_PER_MILE : km);
+}
+
+// distanceUnit is the short label: "km" or "mi".
+export function distanceUnit(): string {
+  return useMiles() ? 'mi' : 'km';
+}
+
+// formatDistance is value + unit, thousands-separated: "12 345 km".
+export function formatDistance(km: number): string {
+  return `${distanceValue(km).toLocaleString()} ${distanceUnit()}`;
+}