Expand frontend unit test coverage

[karutoil]

Summary

The frontend currently has only 1 unit test file (src/__tests__/rbac-permissions.test.tsx) covering permission helpers. The rest of the frontend — hooks, stores, API services, and utility functions — has zero unit test coverage.

Current State

• Vitest configured with jsdom environment and @testing-library/jest-dom
• 1 test file: rbac-permissions.test.tsx (~460 lines)
• Frontend tests are NOT run in CI (jsdom not installed in CI environment)
• Playwright E2E covers UI behavior (functional.spec.ts, 1340 lines) but is slow and not granular
• ~32 custom hooks, 3 Zustand stores, 23 API service files — all untested

Priority Test Targets

1. Zustand stores (authStore, themeStore, backupDownloadStore) — state transitions, persistence, side effects
2. Custom hooks (useServer, useServers, useNodes, useFileManager, useSseConsole) — query key usage, refetch intervals, error handling
3. API client (services/api/client.ts) — 401 interceptor, CSRF handling, error reporting
4. Utility functions — query key factory (queryKeys.ts), validators
5. Plugin system (plugins/PluginProvider.tsx, plugins/loader.ts) — manifest loading, dynamic imports

Acceptance Criteria

• Unit tests for all 3 Zustand stores
• Unit tests for at least 10 priority hooks
• Unit tests for API client (interceptors, CSRF, error paths)
• Frontend Vitest added to CI pipeline (jsdom dependency added)
• Test infrastructure: shared test utilities (mock API client, mock stores, test wrappers)
• Coverage threshold set (e.g., 50% for hooks/stores as initial target)