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/main.ts

@@ -3,28 +3,15 @@ import type { StorybookConfig } from '@storybook/react-vite';
 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',
     options: {},
   },
-  docs: {
-    autodocs: 'tag',
-  },
   staticDirs: ['../brandassets'],
   viteFinal: async (config) => {
-    // Inherit aliases from vite.config.ts automatically via react-vite framework
     return config;
   },
 };