test: add Vitest setup with 115 tests (backend + frontend)

frontend/vitest.config.ts

@@ -0,0 +1,21 @@
+import { defineConfig } from 'vitest/config';
+import react from '@vitejs/plugin-react';
+import { resolve } from 'path';
+
+export default defineConfig({
+  plugins: [react()],
+  resolve: {
+    alias: {
+      '@': resolve(__dirname, 'src'),
+    },
+  },
+  test: {
+    globals: true,
+    environment: 'jsdom',
+    include: ['tests/**/*.test.ts'],
+    coverage: {
+      provider: 'v8',
+      include: ['src/canvas/**', 'src/history/**'],
+    },
+  },
+});