test: add Vitest configuration and setup

- Add vitest.config.ts with jsdom environment and path aliases
- Add tests/setup.ts with mocks for Tauri API, crypto, and localStorage
- Add test:coverage script to package.json

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

desktop/vitest.config.ts

@@ -0,0 +1,36 @@
+import { defineConfig } from 'vitest/config';
+import react from '@vitejs/plugin-react';
+import path from 'path';
+
+export default defineConfig({
+  plugins: [react()],
+  test: {
+    globals: true,
+    environment: 'jsdom',
+    setupFiles: [path.resolve(__dirname, './tests/setup.ts')],
+    include: ['tests/**/*.test.ts', 'tests/**/*.test.tsx'],
+    exclude: ['tests/e2e/**', 'node_modules/**'],
+    coverage: {
+      provider: 'v8',
+      reporter: ['text', 'json', 'html'],
+      exclude: [
+        'node_modules/**',
+        'tests/**',
+        '**/*.d.ts',
+        '**/*.config.*',
+        'src/main.tsx',
+      ],
+      thresholds: {
+        lines: 60,
+        functions: 60,
+        branches: 60,
+        statements: 60,
+      },
+    },
+  },
+  resolve: {
+    alias: {
+      '@': path.resolve(__dirname, './src'),
+    },
+  },
+});