chore(test): add vitest runner + smoke test

No test runner existed in the repo. Add vitest (node env, *.spec.ts
discovery) with a minimal config mirroring only the `@`→src alias, plus
`test`/`test:watch` scripts and a smoke test as a known-good baseline.

Precursor for the nostr-patterns review fixes (events store coordinate
keying #121, monotonic created_at #122), which ship with unit tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

vitest.config.ts

@@ -0,0 +1,23 @@
+import { fileURLToPath, URL } from 'node:url'
+import { defineConfig } from 'vitest/config'
+
+// Minimal test runner config. Unit tests live next to the code they
+// cover as `*.spec.ts`. The default `node` environment is enough for
+// the pure logic + Pinia/Vue-reactivity tests we run today (no DOM);
+// switch a given file to jsdom via a per-file `// @vitest-environment`
+// pragma if a component test ever needs it.
+//
+// Only the bare `@` → src alias is mirrored from vite.config.ts. The
+// brand-kit aliases (@brand-*) are build-time asset shims that unit
+// tests don't touch, so they're deliberately omitted to keep this lean.
+export default defineConfig({
+  test: {
+    environment: 'node',
+    include: ['src/**/*.spec.ts'],
+  },
+  resolve: {
+    alias: [
+      { find: '@', replacement: fileURLToPath(new URL('./src', import.meta.url)) },
+    ],
+  },
+})