Parse lastStamp as Date in AvailabilityService

Type AvailabilityInfo.lastStamp as Date and convert from the JSON
string in the service after fetch, so consumers work with real
Date objects.

client/src/availability.ts

@@ -67,8 +67,8 @@ export class Availability {
     var dotClass = entry.loggedIn ? 'tapi-dot-on' : 'tapi-dot-off'
     var time = ''
     if (entry.lastStamp) {
-      var d = new Date(entry.lastStamp)
       var pad = (n: number) => n.toString().padStart(2, '0')
+      var d = entry.lastStamp
       time = pad(d.getDate()) + '.' + pad(d.getMonth() + 1) + '. ' + pad(d.getHours()) + ':' + pad(d.getMinutes())
     }
     var location = ''