Add Storybook addons (a11y, storysource) and Playwright MCP

- @storybook/addon-a11y: real-time axe-core accessibility panel per story
- @storybook/addon-storysource: story source code panel in addons bar
- @playwright/mcp: browser automation for visual screenshot feedback loop

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -32,8 +32,11 @@
   },
   "devDependencies": {
     "@eslint/js": "^9.39.4",
+    "@playwright/mcp": "^0.0.68",
+    "@storybook/addon-a11y": "^8.6.14",
     "@storybook/addon-designs": "^8.0.0",
     "@storybook/addon-essentials": "^8.4.0",
+    "@storybook/addon-storysource": "^8.6.14",
     "@storybook/blocks": "^8.4.0",
     "@storybook/react": "^8.4.0",
     "@storybook/react-vite": "^8.4.0",