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/package.json

@@ -11,14 +11,23 @@
     "test:e2e": "playwright test"
   },
   "dependencies": {
+    "@hookform/resolvers": "^3.9.0",
     "@ketopath/auth": "workspace:*",
     "@ketopath/shared": "workspace:*",
     "@ketopath/ui": "workspace:*",
+    "@radix-ui/react-label": "^2.1.0",
+    "@radix-ui/react-slot": "^1.1.0",
     "better-auth": "^1.0.21",
+    "class-variance-authority": "^0.7.0",
+    "clsx": "^2.1.1",
+    "lucide-react": "^0.439.0",
     "next": "^14.2.5",
     "next-intl": "^3.17.2",
     "react": "^18.3.1",
-    "react-dom": "^18.3.1"
+    "react-dom": "^18.3.1",
+    "react-hook-form": "^7.53.0",
+    "tailwind-merge": "^2.5.2",
+    "tailwindcss-animate": "^1.0.7"
   },
   "devDependencies": {
     "@ketopath/eslint-config": "workspace:*",