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>

.storybook/main.ts

@@ -4,7 +4,16 @@ const config: StorybookConfig = {
   stories: ['../src/**/*.stories.@(ts|tsx)'],
   addons: [
     '@storybook/addon-essentials',
+    '@storybook/addon-a11y',
     '@storybook/addon-designs',
+    {
+      name: '@storybook/addon-storysource',
+      options: {
+        loaderOptions: {
+          injectStoryParameters: true,
+        },
+      },
+    },
   ],
   framework: {
     name: '@storybook/react-vite',