Add draft-only invoicing rules, send reminders, and time clock billing.

Restrict line item edits to draft invoices, auto-create drafts on clock-out,
and add sendReminderAt scheduling with dashboard due reminders.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/lib/draft-invoice.ts

@@ -0,0 +1,11 @@
+/** Default invoice number format (matches web/mobile create forms). */
+export function generateInvoiceNumber(now = new Date()): string {
+  const date = now.toISOString().slice(0, 10).replace(/-/g, "");
+  return `INV-${date}-${String(now.getTime()).slice(-6)}`;
+}
+
+export function defaultDueDate(issueDate: Date): Date {
+  const due = new Date(issueDate);
+  due.setDate(due.getDate() + 30);
+  return due;
+}