Add lastStamp to availability and show it on people tiles

Server query extended with MAX(BU_BU) per user as LAST_STAMP,
exposed via the Availability model. Client formats it as
DD.MM. HH:mm next to the loggedIn dot, replacing the mocked time.

client/src/availability-info.ts

@@ -2,4 +2,5 @@ export class AvailabilityInfo {
   public user: string;
   public loggedIn: boolean;
   public extension: string;
+  public lastStamp: string;
 }