Stage 3: Complete expense pages + reports

- Add Expense CRUD pages (/expenses list, /new form, /[id] detail) + actions
- Expense form: date/description/category/amount/receipt, auto-create from purchases
- Reports page: 3 tabs (monthly profit, expense breakdown, cash flow) with tables
- Add Expenses + Reports to AccountsNav tabs

All expense features wired. Reports skeleton ready for data integration.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/components/AccountsNav.tsx

@@ -4,6 +4,8 @@ const TABS = [
   { href: '/admin/accounts', label: 'Ledger' },
   { href: '/admin/accounts/stock', label: 'Stock' },
   { href: '/admin/accounts/purchases', label: 'Purchases' },
+  { href: '/admin/accounts/expenses', label: 'Expenses' },
+  { href: '/admin/accounts/reports', label: 'Reports' },
 ] as const;
 
 export default function AccountsNav({ active }: { active: (typeof TABS)[number]['href'] }) {