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

client/src/components/Map/RouteCalculator.ts

@@ -240,7 +240,9 @@ export async function calculateRouteWithLegs(
   }
 
   const coords = waypoints.map((p) => `${p.lng},${p.lat}`).join(';')
-  const cacheKey = `${profile}:${coords}`
+  // The cached result carries formatted leg distances, so the active distance unit is
+  // part of the key — otherwise switching km↔mi would return stale text (#1300).
+  const cacheKey = `${profile}:${getDistanceUnit()}:${coords}`
   const cached = routeCache.get(cacheKey)
   if (cached) return cached