feat: add Vitest unit testing infrastructure with initial test suite.

vitest.config.ts

@@ -0,0 +1,59 @@
+/// <reference types="vitest" />
+import { defineConfig } from 'vite'
+import react from '@vitejs/plugin-react'
+
+export default defineConfig({
+  plugins: [react()],
+  test: {
+    // Use jsdom environment for DOM testing
+    environment: 'jsdom',
+    
+    // Global test setup
+    setupFiles: ['./src/test/setup.ts'],
+    
+    // Include unit test files co-located with source
+    include: [
+      'src/**/*.{test,spec}.{js,mjs,cjs,ts,mts,cts,jsx,tsx}'
+    ],
+    
+    // Exclude directories
+    exclude: [
+      'node_modules',
+      'dist',
+      '.git',
+      '.cache',
+      'tests/' // Exclude integration/e2e folder for now
+    ],
+    
+    // Enable global test functions (describe, it, expect)
+    globals: true,
+    
+    // Coverage configuration
+    coverage: {
+      provider: 'v8',
+      reporter: ['text', 'json', 'html'],
+      exclude: [
+        'node_modules/',
+        'src/test/',
+        '**/*.d.ts',
+        '**/*.config.ts',
+        'src/main.tsx',
+        'src/vite-env.d.ts'
+      ],
+      thresholds: {
+        global: {
+          branches: 70,
+          functions: 70,
+          lines: 70,
+          statements: 70
+        }
+      }
+    },
+    
+    // Test timeout
+    testTimeout: 10000,
+    
+    // Retry failed tests once
+    retry: 1
+  }
+})