Upgrade to React 19, MUI v7, Storybook 9

- React 18 → 19, MUI v5 → v7, Storybook 8 → 9
- Fix Grid v2 API in Footer (remove item prop, use size prop)
- Inline provider fixtures (was importing from excluded demo dir)
- Remove consolidated SB addons (essentials, storysource, blocks)
- Update addon-designs to SB9-compatible version
- Add autodocs via tags in preview config
- Add Tailwind v3, PostCSS, autoprefixer dev deps (config next)
- Zero TypeScript errors, Storybook starts clean

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.storybook/preview.tsx

@@ -6,6 +6,7 @@ import { theme } from '../src/theme';
 import '../src/theme/generated/tokens.css';
 
 const preview: Preview = {
+  tags: ['autodocs'],
   decorators: [
     (Story) => (
       <ThemeProvider theme={theme}>