Add Next.js client app with Prisma and auth

Introduce a new 'client' Next.js app scaffold: configuration files (next.config.ts, eslint.config.mjs, postcss, tsconfig), Prisma config and schema, app routes including NextAuth API and server actions, session/provider utilities, proxy, public assets, README and .gitignore. Also add IDE project files under .idea and a root README to initialize the repository structure for development.

client/src/lib/prisma.ts

@@ -0,0 +1,22 @@
+import { PrismaClient } from "@/generated/prisma/client";
+import { PrismaPg } from "@prisma/adapter-pg";
+
+const databaseUrl = process.env.DATABASE_URL;
+
+if (!databaseUrl) {
+  throw new Error("Missing DATABASE_URL environment variable.");
+}
+
+const globalForPrisma = globalThis as unknown as {
+  prisma: PrismaClient | undefined;
+};
+
+export const prisma =
+  globalForPrisma.prisma ??
+  new PrismaClient({
+    adapter: new PrismaPg({ connectionString: databaseUrl }),
+  });
+
+if (process.env.NODE_ENV !== "production") {
+  globalForPrisma.prisma = prisma;
+}