feat(frontend): invite-based registration UI

- Add /register route with invite code prefill, password show/hide
- Add /login?registered=1 success banner
- Add /admin/invites page: list, create, revoke, copy link
- Add Einladungen nav section to admin sidebar (ADMIN_USER perm)
- Add invite error codes to errors.ts
- Add 48 i18n keys across de/en/es
- Update hooks.server.ts to allow public access to invite/register API

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

frontend/src/routes/admin/+page.svelte

@@ -46,6 +46,16 @@ onMount(() => {
 			</a>
 		{/if}
 
+		{#if data.canManageUsers}
+			<a href="/admin/invites" class="flex items-center justify-between px-4 py-4 hover:bg-muted">
+				<div>
+					<div class="font-sans text-sm font-bold text-ink">{m.admin_tab_invites()}</div>
+					<div class="mt-0.5 font-sans text-xs text-ink-3">{data.inviteCount}</div>
+				</div>
+				<span class="text-ink-3">›</span>
+			</a>
+		{/if}
+
 		{#if data.canManageTags}
 			<a href="/admin/tags" class="flex items-center justify-between px-4 py-4 hover:bg-muted">
 				<div>