Add user database service and unit tests

- Implement user database service with functions for user management (create, find, update, delete).
- Add comprehensive unit tests for user database service using Vitest.
- Mock database interactions to ensure isolated testing.
- Create setup files for unit tests to handle database connections and global mocks.
- Introduce error handling for unique constraints and foreign key violations.
- Enhance logging for better traceability during database operations.

vite.config.ts

@@ -1,5 +1,4 @@
 // vite.config.ts
-/// <reference types="vitest" />
 import path from 'path';
 import { defineConfig } from 'vitest/config';
 import react from '@vitejs/plugin-react';
@@ -40,7 +39,7 @@ export default defineConfig({
     fileParallelism: false,
     environment: 'jsdom',
     globalSetup: './src/tests/setup/global-setup.ts',
-    setupFiles: ['./src/tests/setup/unit-setup.ts'],
+    setupFiles: ['./src/tests/setup/tests-setup-unit.ts'],
     // Explicitly include all test files that are NOT integration tests.
     include: ['src/**/*.test.{ts,tsx}'],
     // Exclude integration tests and other non-test files from the unit test runner.