fix(admin): rename users edit default action to named update

SvelteKit 2 forbids mixing a `default` action with named actions; the
page also exports a `delete` action. Posting the edit form therefore
returned a 500 with "When using named actions, the default action
cannot be used." Rename the action to `update` and point the form
at `?/update`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/routes/admin/users/[id]/+page.svelte

@@ -84,6 +84,7 @@ $effect(() => {
 		<form
 			id="edit-user-form"
 			method="POST"
+			action="?/update"
 			use:enhance
 			oninput={unsaved.markDirty}
 			class="space-y-5"