fix(members): create invite on first click when no active invite exists

When activeInvite is null and the user clicks the invite card, POST to
/members/invites first to generate a code, then toggle the panel open.

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

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

@@ -59,6 +59,16 @@
 		}
 	}
 
+	async function handleInviteClick() {
+		if (!activeInvite) {
+			const res = await fetch('/members/invites', { method: 'POST' });
+			if (res.ok) {
+				activeInvite = await res.json();
+			}
+		}
+		showInvitePanel = !showInvitePanel;
+	}
+
 	async function handleRegenerate() {
 		const res = await fetch('/members/invites', { method: 'POST' });
 		if (res.ok) {
@@ -80,7 +90,7 @@
 		showInviteCard={isPlanner}
 		onremove={handleRemove}
 		onrolechange={handleRoleChange}
-		oninviteclick={() => (showInvitePanel = !showInvitePanel)}
+		oninviteclick={handleInviteClick}
 	/>
 
 	{#if showInvitePanel && isPlanner && activeInvite}