Add firma flag to availability and show office/home label

Server query gets the last stamp's BU_TERM in the same OUTER APPLY
as LAST_STAMP, exposing FIRMA (1 = Zeiterfassung/Büro). Three OUTER
APPLYs reduced to two by combining MAX(BU_BU) with TOP 1 ORDER BY.

Client model gains a firma boolean and the people-tile indicator
appends "· Büro" or "· Home" next to the timestamp, but only when
the user is logged in.

client/src/availability-info.ts

@@ -3,4 +3,5 @@ export class AvailabilityInfo {
   public loggedIn: boolean;
   public extension: string;
   public lastStamp: string;
+  public firma: boolean;
 }