Add Vitest + React Testing Library setup for viewer-frontend

Establishes frontend unit testing infra (jsdom env, RTL, jest-dom
matchers, common Radix/jsdom polyfills) and seeds it with tests for
lib/utils.ts, lib/photo-utils.ts, hooks/useFocusTrap.ts, and
ThemeToggle.tsx (46 tests total).

viewer-frontend/vitest.config.ts

@@ -0,0 +1,19 @@
+import path from 'path';
+import react from '@vitejs/plugin-react';
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  plugins: [react()],
+  test: {
+    environment: 'jsdom',
+    globals: true,
+    setupFiles: ['./vitest.setup.ts'],
+    include: ['**/*.test.{ts,tsx}'],
+    exclude: ['node_modules', '.next', 'e2e'],
+  },
+  resolve: {
+    alias: {
+      '@': path.resolve(__dirname, '.'),
+    },
+  },
+});