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>

packages/auth/src/server.ts

@@ -8,9 +8,11 @@ const env = readAuthEnv();
 
 const isGoogleConfigured = Boolean(env.GOOGLE_CLIENT_ID && env.GOOGLE_CLIENT_SECRET);
 
-export const enabledSocialProviders = isGoogleConfigured ? (['google'] as const) : ([] as const);
+export type SocialProvider = 'google';
 
-export type SocialProvider = (typeof enabledSocialProviders)[number];
+export const enabledSocialProviders: ReadonlyArray<SocialProvider> = isGoogleConfigured
+  ? ['google']
+  : [];
 
 const googleProvider = isGoogleConfigured
   ? {