Add tsconfig.storybook.json to cover *.stories.tsx files excluded by
tsconfig.app.json, resolving TS2307 for @storybook/react-vite imports.
Replace 1x1 pixel PNG with a visible red/white checkerboard in the
ImagePreview story.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
