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>

src/components/organisms/Footer/Footer.tsx

@@ -99,7 +99,7 @@ export const Footer = React.forwardRef<HTMLDivElement, FooterProps>(
           {/* Main footer content */}
           <Grid container spacing={{ xs: 4, md: 6 }}>
             {/* Logo + tagline column */}
-            <Grid item xs={12} md={4}>
+            <Grid size={{ xs: 12, md: 4 }}>
               <Box sx={{ mb: 2 }}>{logo}</Box>
               {tagline && (
                 <Typography
@@ -157,10 +157,7 @@ export const Footer = React.forwardRef<HTMLDivElement, FooterProps>(
             {/* Link group columns */}
             {linkGroups.map((group) => (
               <Grid
-                item
-                xs={6}
-                sm={4}
-                md
+                size={{ xs: 6, sm: 4, md: 'grow' }}
                 key={group.heading}
                 component="nav"
                 aria-label={group.heading}