fix: Break circular dependency with shared constants file

Circular dependency was caused by:
- comfyPageFixture.ts importing testComfySnapToGridGridSize from ComfyPage.ts
- ComfyPage.ts re-exporting comfyPageFixture

Solution:
- Extract testComfySnapToGridGridSize to constants.ts
- Both files import from constants.ts
- Only type-import ComfyPage in comfyPageFixture.ts

Now clean dependency graph with no cycles.

browser_tests/fixtures/ComfyPage.ts

@@ -1589,9 +1589,8 @@ export abstract class ComfyPage {
   }
 }
 
-export const testComfySnapToGridGridSize = 50
-
-// Re-export fixture from separate file to avoid circular dependencies
+// Re-export shared constants and fixture
+export { testComfySnapToGridGridSize } from './constants'
 export { comfyPageFixture } from './comfyPageFixture'
 
 const makeMatcher = function <T>(