test: add test infrastructure for frontend and server

- Frontend: vitest.config.ts with vue plugin + jsdom, dummy component test
- Server: in-memory SQLite test DB factory + Hono testClient helper + smoke test
- CI: add pnpm audit and server coverage threshold steps
- Root: vitest workspace config for multi-project test discovery

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

vitest.workspace.ts

@@ -0,0 +1,11 @@
+import { defineWorkspace } from 'vitest/config'
+
+export default defineWorkspace([
+  {
+    test: {
+      name: 'server',
+      include: ['server/src/**/*.test.ts'],
+    },
+  },
+  'frontend/vitest.config.ts',
+])