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/i18n.tsx

@@ -128,6 +128,7 @@ const en: Dict = {
   'mx.tipDxConf': 'Entity confirmed (other callsign)', 'mx.tipDxWork': 'Entity worked (other callsign)',
   'mx.tipNone': 'Never worked', 'mx.tipClick': 'click to list the QSOs',
   'icmp.scopeNoStream': 'This radio does not send its scope over CI-V — its own screen still works.',
+  'gen.miles': 'Distances in miles', 'gen.milesHint': '(instead of kilometres)',
   'qslm.qrzTitle': 'Open this callsign on QRZ.com',
   'qslm.lotwAllCalls': 'All my callsigns',
   'qslm.lotwAllCallsTitle': "Download the confirmations of every callsign on the LoTW account, not just this profile's. A QSO made as F4BPO/P or TM2Q is confirmed at LoTW but never reaches an F4BPO profile without this.",
@@ -623,6 +624,7 @@ const fr: Dict = {
   'mx.tipDxConf': 'Entité confirmée (autre indicatif)', 'mx.tipDxWork': 'Entité contactée (autre indicatif)',
   'mx.tipNone': 'Jamais contacté', 'mx.tipClick': 'cliquer pour lister les QSO',
   'icmp.scopeNoStream': "Cette radio n'envoie pas son scope en CI-V — son propre écran fonctionne toujours.",
+  'gen.miles': 'Distances en miles', 'gen.milesHint': '(au lieu des kilomètres)',
   'qslm.qrzTitle': 'Ouvrir cet indicatif sur QRZ.com',
   'qslm.lotwAllCalls': 'Tous mes indicatifs',
   'qslm.lotwAllCallsTitle': "Télécharger les confirmations de tous les indicatifs du compte LoTW, pas seulement celui du profil. Un QSO fait en F4BPO/P ou TM2Q est confirmé chez LoTW mais n'atteint jamais un profil F4BPO sans cette option.",