feat(web): adopt shadcn/ui primitives for auth surface

Add shadcn/ui plumbing:
- components.json (style: new-york, baseColor: slate, primary: green ~142 71%)
- src/lib/utils.ts with cn() helper (clsx + tailwind-merge)
- Tailwind config switched to CSS variables theming with primary/destructive/
  muted/card etc. tokens, dark-mode class, tailwindcss-animate plugin
- src/styles/globals.css declares :root and .dark variable palettes

Initial component set under src/components/ui:
- Button (cva variants: default/destructive/outline/secondary/ghost/link, asChild)
- Input
- Label (Radix Label)
- Card + CardHeader/Title/Description/Content/Footer

Refactor existing auth surface to the new primitives:
- sign-in / sign-up pages wrap form in a Card with Title/Description (+ footer
  for sign-up disclaimer)
- sign-in-form / sign-up-form use Input, Label, Button; tokens replace bespoke
  emerald/slate classes; divider and "Continua con Google" use the same Button
- Home page CTAs become Button asChild around Link; SignOutButton uses Button
- Hide-Google logic still works: enabledSocialProviders is now
  ReadonlyArray<SocialProvider> for ergonomic .includes() checks

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/tailwind.config.ts

@@ -1,15 +1,62 @@
 import type { Config } from 'tailwindcss';
+import animate from 'tailwindcss-animate';
 
 const config: Config = {
+  darkMode: ['class'],
   content: ['./src/**/*.{ts,tsx}', '../../packages/ui/src/**/*.{ts,tsx}'],
   theme: {
+    container: {
+      center: true,
+      padding: '2rem',
+      screens: { '2xl': '1400px' },
+    },
     extend: {
       fontFamily: {
         sans: ['var(--font-sans)', 'system-ui', 'sans-serif'],
       },
+      colors: {
+        border: 'hsl(var(--border))',
+        input: 'hsl(var(--input))',
+        ring: 'hsl(var(--ring))',
+        background: 'hsl(var(--background))',
+        foreground: 'hsl(var(--foreground))',
+        primary: {
+          DEFAULT: 'hsl(var(--primary))',
+          foreground: 'hsl(var(--primary-foreground))',
+        },
+        secondary: {
+          DEFAULT: 'hsl(var(--secondary))',
+          foreground: 'hsl(var(--secondary-foreground))',
+        },
+        destructive: {
+          DEFAULT: 'hsl(var(--destructive))',
+          foreground: 'hsl(var(--destructive-foreground))',
+        },
+        muted: {
+          DEFAULT: 'hsl(var(--muted))',
+          foreground: 'hsl(var(--muted-foreground))',
+        },
+        accent: {
+          DEFAULT: 'hsl(var(--accent))',
+          foreground: 'hsl(var(--accent-foreground))',
+        },
+        popover: {
+          DEFAULT: 'hsl(var(--popover))',
+          foreground: 'hsl(var(--popover-foreground))',
+        },
+        card: {
+          DEFAULT: 'hsl(var(--card))',
+          foreground: 'hsl(var(--card-foreground))',
+        },
+      },
+      borderRadius: {
+        lg: 'var(--radius)',
+        md: 'calc(var(--radius) - 2px)',
+        sm: 'calc(var(--radius) - 4px)',
+      },
     },
   },
-  plugins: [],
+  plugins: [animate],
 };
 
 export default config;