Add Vitest JavaScript testing framework for SPA

Set up comprehensive JavaScript testing infrastructure to ensure SPA
implementation matches traditional Rails scoring interface.

Testing Strategy:
- Backend API tests: Expand existing Rails controller tests (~150 lines)
- System tests: Browser integration with Capybara/Selenium (~200 lines)
- JavaScript unit tests: Test offline behavior with Vitest (~500 lines)

Setup:
- Initialize npm with package.json
- Install Vitest, jsdom, fake-indexeddb for testing
- Configure Vitest with jsdom environment
- Create test setup with Rails helper mocks
- Add 12 passing tests for HeatDataManager (IndexedDB operations)

Test Coverage:
- Dirty scores management (create, update, remove, clear)
- Last-write-wins for duplicate heat/slot scores
- Batch upload to server with success/failure handling
- Network error handling
- Data fetching from server

Benefits:
- Fast test execution (Vitest uses Vite transformation)
- Jest-compatible API (familiar, lots of examples)
- Native ESM support (works with Rails import maps)
- Built-in browser API mocks (IndexedDB via fake-indexeddb)
- Systematic conversion of Ruby tests to JS ensures feature parity

All tests passing:
  Test Files  1 passed (1)
       Tests  12 passed (12)
    Duration  323ms

Updated plans/SPA_SYNC_STRATEGY.md with complete testing strategy and
CI configuration examples for GitHub Actions and GitLab CI.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: rubys