Rename firma to atCompany on server and client

Renames the Zeiterfassung-vs-home flag to a clearer English name
(SQL alias AT_COMPANY, model property AT_COMPANY, JSON atCompany)
in both the server query/model and the client AvailabilityInfo
type and people-tile renderer.

client/src/availability-info.ts

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