feat(01-02): login page UI, logout flow, seed script, and auth unit tests

- src/app/(auth)/layout.tsx: centered auth layout for login page
- src/app/(auth)/login/page.tsx: login form with error/loading states, sign up link
- src/components/providers.tsx: SessionProvider wrapper for client-side session
- src/components/layout/header.tsx: authenticated header with Sign out button
- src/app/(dashboard)/layout.tsx: dashboard layout wrapping Header component
- src/app/(dashboard)/dashboard/page.tsx: basic dashboard page post-login
- src/app/layout.tsx: wrap root with SessionProvider via Providers component
- prisma/seed.ts: idempotent seed for Demo ISP tenant + admin + super-admin users
- package.json: add db:seed script and prisma.seed config, add tsx devDep
- src/lib/__tests__/auth.test.ts: 8 unit tests for authOptions callbacks

package.json

@@ -11,7 +11,11 @@
     "test:watch": "vitest",
     "db:push": "prisma db push",
     "db:generate": "prisma generate",
-    "db:studio": "prisma studio"
+    "db:studio": "prisma studio",
+    "db:seed": "npx tsx prisma/seed.ts"
+  },
+  "prisma": {
+    "seed": "npx tsx prisma/seed.ts"
   },
   "dependencies": {
     "@prisma/client": "^6.19.2",
@@ -33,6 +37,7 @@
     "eslint": "^9",
     "eslint-config-next": "16.1.6",
     "tailwindcss": "^4",
+    "tsx": "^4.21.0",
     "typescript": "^5",
     "vitest": "^4.0.18"
   }