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/uiPref.ts

@@ -40,6 +40,7 @@ const PORTABLE_KEYS = [
   'opslog.clusterFilterSource', 'opslog.clusterGroup', 'opslog.clusterBands',
   'opslog.clusterLockBand', 'opslog.clusterLockMode', 'opslog.clusterStatusFilter',
   'opslog.clusterModeFilter', 'opslog.clusterSearch', 'opslog.clusterHideWorked',
+  'opslog.distanceMiles', // distances shown in statute miles rather than km
   'opslog.activeTab',     // last selected tab
   'opslog.mainSplit',    // Main tab: width share of the left pane (percent) — legacy, read once to seed mainShares
   'opslog.mainShares',   // Main tab: column shares per column count, as {2:[..],3:[..],4:[..]}