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/components.json

@@ -0,0 +1,21 @@
+{
+  "$schema": "https://ui.shadcn.com/schema.json",
+  "style": "new-york",
+  "rsc": true,
+  "tsx": true,
+  "tailwind": {
+    "config": "tailwind.config.ts",
+    "css": "src/styles/globals.css",
+    "baseColor": "slate",
+    "cssVariables": true,
+    "prefix": ""
+  },
+  "aliases": {
+    "components": "@/components",
+    "utils": "@/lib/utils",
+    "ui": "@/components/ui",
+    "lib": "@/lib",
+    "hooks": "@/hooks"
+  },
+  "iconLibrary": "lucide"
+}