Add multi-line sale builder with pending-draft safeguard

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

src/routes/invoices/[id]/+page.server.js

@@ -0,0 +1,10 @@
+import { error } from '@sveltejs/kit';
+import { getInvoice } from '$lib/server/invoices.js';
+
+export function load({ params }) {
+  const result = getInvoice(params.id);
+  if (!result || result.invoice.status !== 'saved') {
+    throw error(404, 'Invoice not found');
+  }
+  return result;
+}