fix(members): add error toasts for invite failures + Content-Type header

- handleInviteClick: show toast and bail early when POST /invites fails
- handleRegenerate: show toast when regeneration POST fails
- handleRoleChange: add Content-Type: application/json header on PATCH

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/(app)/members/+page.svelte

@@ -31,6 +31,7 @@
 
 		const res = await fetch('/members/' + member.userId, {
 			method: 'PATCH',
+			headers: { 'Content-Type': 'application/json' },
 			body: JSON.stringify({ role: newRole })
 		});
 
@@ -64,6 +65,9 @@
 			const res = await fetch('/members/invites', { method: 'POST' });
 			if (res.ok) {
 				activeInvite = await res.json();
+			} else {
+				showToast('Einladung konnte nicht erstellt werden');
+				return;
 			}
 		}
 		showInvitePanel = !showInvitePanel;
@@ -73,6 +77,8 @@
 		const res = await fetch('/members/invites', { method: 'POST' });
 		if (res.ok) {
 			activeInvite = await res.json();
+		} else {
+			showToast('Link konnte nicht erneuert werden');
 		}
 	}
 </script>