fix: address review feedback on the distance unit setting

- server: allow distance_unit as an admin default (+ value validation) so the
  Admin "Default User Settings" toggle persists instead of returning 400
- i18n: add settings.distance to all 20 locales and translate the labels
  through t() instead of hardcoding "Distance"
- route legs: include the unit in the OSRM cache key and recompute on a unit
  switch, so map and sidebar distances refresh and never mix units
- keep wind speed tied to the temperature unit — a distance setting must not
  silently flip existing Fahrenheit users from mph to km/h
- restore the sub-1km metres reading for metric, convert GPX elevation to feet
  for imperial, and format distances with a '.' decimal in every locale
- add units.test.ts

shared/src/i18n/ja/settings.ts

@@ -52,6 +52,7 @@ const settings: TranslationStrings = {
   'settings.auto': '自動',
   'settings.language': '言語',
   'settings.temperature': '温度単位',
+  'settings.distance': '距離単位',
   'settings.timeFormat': '時刻形式',
   'settings.bookingLabels': '予約ルートのラベル',
   'settings.bookingLabelsHint': '地図に駅・空港名を表示。オフ時はアイコンのみ。',