Replace cost unit endpoints for staff member

• cost unit changes are made by patch staff
• so no separate cost unit endpoints anymore

apps/frontend/src/app/staff-admin/cost-units/edit-cost-unit-dialog.component.ts

 import { MAT_DIALOG_DATA } from '@angular/material/dialog';
-import { Component, Inject } from '@angular/core';
+import {
+  AfterViewInit, Component, ElementRef, Inject, ViewChild
+} from '@angular/core';
 import { FormGroup, FormBuilder, Validators } from '@angular/forms';
 
 @Component({
@@ -9,7 +11,7 @@ import { FormGroup, FormBuilder, Validators } from '@angular/forms';
       <mat-dialog-content fxLayout="column">
         <form [formGroup]="editCostUnitForm" fxLayout="column">
           <p>
-            Die Nummer der Kostenstelle besteht aus 5 Ziffern.
+            Die Nummer der Kostenstelle besteht aus 3 Ziffern (ggf. führende Nullen).
           </p>
           <mat-form-field>
             <input matInput formControlName="key" type="text" placeholder="Titel" [value]="data.key"/>
@@ -30,10 +32,11 @@ import { FormGroup, FormBuilder, Validators } from '@angular/forms';
       </mat-dialog-actions>
   `
 })
-export class EditCostUnitDialogComponent {
+export class EditCostUnitDialogComponent implements AfterViewInit {
   editCostUnitForm: FormGroup;
   saveButtonLabel: string;
   title: string;
+  @ViewChild(HTMLInputElement) firstInputElement?: ElementRef;
 
   constructor(private fb: FormBuilder,
               @Inject(MAT_DIALOG_DATA) public data: any) {
@@ -41,8 +44,14 @@ export class EditCostUnitDialogComponent {
     this.title = data.new ? 'Kostenstelle neu anlegen' : 'Kostenstelle ändern';
     this.editCostUnitForm = this.fb.group({
       key: this.fb.control(this.data.key,
-        [Validators.required, Validators.pattern(/^[0-9]{5}$/)]),
+        [Validators.required, Validators.pattern(/^[0-9]{3}$/)]),
       label: this.fb.control(this.data.label, [Validators.required])
     });
   }
+
+  ngAfterViewInit(): void {
+    setTimeout(() => {
+      if (this.firstInputElement) this.firstInputElement.nativeElement.focus();
+    });
+  }
 }