fix: validate tenant and lease contact details

frontend/src/app/app.ts

@@ -41,6 +41,7 @@ export class App implements OnInit {
   readonly apartments = signal<Apartment[]>([]);
   readonly tenants = signal<Tenant[]>([]);
   readonly leases = signal<Lease[]>([]);
+  readonly tenantAttachmentError = signal('');
   private readonly _isLoggedIn = signal<boolean>(false);
   private readonly _username = signal<string>('');
   private readonly _email = signal<string>('');
@@ -179,9 +180,13 @@ export class App implements OnInit {
   }
 
   uploadTenantAttachment(request: { tenantId: string; file: File }): void {
+    this.tenantAttachmentError.set('');
     this.tenantService.uploadAttachment(request.tenantId, request.file).subscribe({
       next: () => this.loadTenants(),
-      error: (err) => console.error('Error uploading tenant attachment', err),
+      error: (error: unknown) => {
+        console.error('Error uploading tenant attachment', error);
+        this.tenantAttachmentError.set(this.errorMessage(error, 'Impossible d’envoyer cette pièce jointe.'));
+      },
     });
   }
 
@@ -234,4 +239,15 @@ export class App implements OnInit {
     link.click();
     URL.revokeObjectURL(url);
   }
+
+  private errorMessage(error: unknown, fallback: string): string {
+    if (typeof error !== 'object' || error === null || !('error' in error)) {
+      return fallback;
+    }
+    const response = error.error;
+    if (typeof response !== 'object' || response === null || !('message' in response)) {
+      return fallback;
+    }
+    return typeof response.message === 'string' ? response.message : fallback;
+  }
 }