feat: implement user registration functionality

- Created registerUser server action with email/password validation
- Implemented password hashing with bcrypt (10 rounds)
- Added email uniqueness check to prevent duplicate registrations
- Created session after successful registration using NextAuth
- Added comprehensive validation for email format and password length (min 8 chars)
- Added email and password validation utilities to lib/validation.ts
- Removed PrismaAdapter from auth.ts (using JWT sessions only)
- Added vitest testing framework and configuration
- Created comprehensive unit tests for registration functionality
- All tests passing (6/6)

Requirements covered: 1.1, 1.2, 1.3, 1.4, 1.5

vitest.config.ts

@@ -0,0 +1,16 @@
+import { defineConfig } from 'vitest/config'
+import path from 'path'
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: 'node',
+    setupFiles: [],
+  },
+  resolve: {
+    alias: {
+      '@': path.resolve(__dirname, './'),
+    },
+    conditions: ['node', 'default'],
+  },
+})