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/comfyPageFixture.ts

@@ -2,8 +2,8 @@ import { test as base } from '@playwright/test'
 
 import { NodeBadgeMode } from '../../src/types/nodeSource'
 import type { ComfyPage } from './ComfyPage'
-import { testComfySnapToGridGridSize } from './ComfyPage'
 import { ComfyMouse } from './ComfyMouse'
+import { testComfySnapToGridGridSize } from './constants'
 import { LocalhostComfyPage } from './LocalhostComfyPage'
 
 /**