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/App.tsx

@@ -57,6 +57,7 @@ import {
 import { Combobox } from '@/components/ui/combobox';
 import { applyAwardRefs, parseAwardRefs as parseManualRefs, spotRefList , withIOTARef, withRDARef } from '@/lib/awardRefs';
 import { ListRadios, SetActiveRadio } from '../wailsjs/go/main/App';
+import { formatDistance } from '@/lib/units';
 import { EventsOn, BrowserOpenURL, WindowMinimise, WindowToggleMaximise, WindowIsMaximised, Quit } from '../wailsjs/runtime/runtime';
 import type { adif as adifModels, lookup as lookupModels, cat as catModels } from '../wailsjs/go/models';
 import type { QSOForm, WorkedBeforeView, StationSettingsForm, ListsSettingsForm, ModePresetForm } from '@/types';
@@ -6419,7 +6420,7 @@ export default function App() {
                     disabled={disabled}
                     onClick={() => p && goto(p.bearingShort, 'SP')}
                     title={p
-                      ? `Rotate short-path · ${Math.round(p.distanceShort).toLocaleString()} km`
+                      ? `Rotate short-path · ${formatDistance(p.distanceShort)}`
                       : (station.my_grid ? 'No remote grid' : 'Set your station grid in Preferences')}
                     className={cn(
                       'inline-flex items-center gap-1 px-2 py-0.5 transition-colors',
@@ -6435,7 +6436,7 @@ export default function App() {
                     type="button"
                     disabled={disabled}
                     onClick={() => p && goto(p.bearingLong, 'LP')}
-                    title={p ? `Rotate long-path · ${Math.round(p.distanceLong).toLocaleString()} km` : ''}
+                    title={p ? `Rotate long-path · ${formatDistance(p.distanceLong)}` : ''}
                     className={cn(
                       'px-1.5 py-0.5 border-l border-info-border text-[10px] transition-colors',
                       disabled
@@ -8570,7 +8571,7 @@ export default function App() {
                     "1.5k" and then appended the unit, giving "1.5kkm" — and even
                     written correctly, "1.5k km" makes a reader do arithmetic to
                     recover a number that was four characters long to begin with. */}
-                <span className="font-mono opacity-80">{o.median_km} km</span>
+                <span className="font-mono opacity-80">{formatDistance(o.median_km)}</span>
                 {/* Out of season is the one an operator must not learn last, so it
                     earns a mark on the badge rather than a line in the tooltip. */}
                 {!o.in_season && <span className="opacity-90">!</span>}