feat: group thousands in inventory item price input

Show the قیمت (تومان) field with thousand separators as the user types
(e.g. 1200000 → 1,200,000). Stored value stays digit-only, so the
Toman→Rial conversion on submit is unaffected.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

assets/admin/pages/InventoryPage.test.tsx

@@ -62,6 +62,11 @@ describe('InventoryPage', () => {
     expect(await screen.findByText('افزودن کالای جدید')).toBeInTheDocument();
     // the source's distinctive "هشدار اتمام" field is present
     expect(screen.getByText('هشدار اتمام')).toBeInTheDocument();
+
+    // price input groups thousands as the user types
+    const price = screen.getByPlaceholderText('قیمت') as HTMLInputElement;
+    fireEvent.change(price, { target: { value: '1200000' } });
+    expect(price.value).toBe('1,200,000');
   });
 
   it('switches to the packages tab and lists a package with its price', async () => {