add tsconfig for storybook files and fix ImagePreview story

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>

clients/web/tsconfig.json

@@ -2,6 +2,7 @@
   "files": [],
   "references": [
     { "path": "./tsconfig.app.json" },
-    { "path": "./tsconfig.node.json" }
+    { "path": "./tsconfig.node.json" },
+    { "path": "./tsconfig.storybook.json" }
   ]
 }