feat(product-dialog): conform aan dialog-pattern + entity-profile

Story 2 van PBI "Alle dialogen conform docs/patterns/dialog.md".

- lib/schemas/product.ts — gedeeld zod-schema (Dialog API)
- actions/products.ts — createProductAction/updateProductAction returnen
  nu code+fieldErrors voor 422-validatie en code: 403 voor demo/auth
- ProductDialog adopt useDirtyCloseGuard, useDialogSubmitShortcut,
  entityDialog* layout-classes; 422-fieldErrors mappen naar form.setError
- docs/specs/dialogs/product.md — entity-profile

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

__tests__/actions/products.test.ts

@@ -107,7 +107,10 @@ describe('createProductAction', () => {
 
     const result = await createProductAction(VALID_DATA)
 
-    expect(result).toMatchObject({ error: expect.stringContaining('gebruik') })
+    expect(result).toMatchObject({
+      code: 422,
+      fieldErrors: { code: expect.arrayContaining([expect.stringContaining('gebruik')]) },
+    })
     expect(mockTransaction).not.toHaveBeenCalled()
   })