chore(client): add lint, format and test tooling

Configure ESLint 10 (flat, Vue + a11y), Prettier, Vitest + @vue/test-utils + jsdom, and add npm scripts for lint/format/test. Document VITE_API_BASE_URL.

client/vitest.config.ts

@@ -0,0 +1,11 @@
+import vue from '@vitejs/plugin-vue'
+import { defineConfig } from 'vitest/config'
+
+export default defineConfig({
+  plugins: [vue()],
+  test: {
+    environment: 'jsdom',
+    globals: true,
+    include: ['src/**/*.spec.ts'],
+  },
+})