v0.3.9 - Hard and extreme difficulties

What's changed

• Added Hard and Extreme computer difficulty levels.
• Hard difficulty now scores line extensions and board pressure instead of only choosing nearby cells.
• Extreme difficulty adds stronger defensive weighting and a one-step opponent response estimate.
• Added English and Chinese labels for the new difficulty levels.
• Updated README, roadmap, changelog, package metadata, and tests.

Verification

• Node syntax checks passed for src/engine.js and src/main.js.
• Node test suite passed: 26 tests.
• git diff --check passed.
• GitHub Actions CI and Pages deployment passed.

Note: the in-app browser could not open the local preview due net::ERR_BLOCKED_BY_CLIENT; UI behavior is covered by source and engine regression tests.