Add E2E and integration test coverage

[apetresc]

Problem

Sabaki's test suite only covers file format parsing and game sorting — all pure JS unit tests. There is no testing of:

• App startup and rendering — a broken import silently produces a black screen with all unit tests passing
• IPC communication between main and renderer processes
• GTP engine integration — attaching engines, sending commands, receiving analysis
• File loading through the UI — opening SGF files end-to-end

Proposed testing additions

1. End-to-end tests with Playwright (Electron support)

Playwright supports launching Electron apps directly via electron.launch(). This enables:

• Smoke test: App launches, renderer loads, Goban component renders
• File loading: Open an SGF file via IPC, verify the board state renders correctly
• Black-screen detection: Assert that the renderer's root component mounts and is visible
• Window lifecycle: Maximize, minimize, close without crash
• Multi-window: Open multiple windows, verify independence

2. GTP engine integration tests

The codebase already has test/engines/resignEngine.js (a minimal GTP engine using @sabaki/gtp's Engine class). We can use this to test:

• EngineSyncer lifecycle: Start, sync board state, stop, and restart
• Command queuing: Send GTP commands and verify responses
• Board state sync: Load a game tree, sync it to the engine, verify the engine received the correct moves
• Analysis parsing: Feed mock info lines and verify analysis state updates
• Error handling: Engine crash, timeout, malformed responses

3. Renderer integration tests

Using Playwright's Electron support to test renderer-side behavior:

• File drag-and-drop: Simulate dropping an SGF file onto the window
• Navigation: Move forward/backward through game moves, verify board updates
• Game tree interaction: Switch between variations

Technical approach

• Framework: Playwright (@playwright/test) with its built-in _electron module for Electron app testing
• GTP tests: Can run as standard Mocha tests by importing EngineSyncer directly and spawning the resign engine
• Test data: Use existing fixtures in test/sgf/, etc.
• CI: Playwright Electron tests require a display server on Linux (xvfb-run) but work natively on macOS/Windows

Non-goals

• Testing pure utility functions (already possible, low value relative to above)
• Visual regression testing (future work)
• Performance benchmarks