settings: Playwright headless toggle + lower agent max_turns

[gabelul]

Ports two upstream-inspired follow-ups in a fork-native way:

• Adds per-project runtime setting playwright_headless (stored in project agent_system.db), surfaced in Settings → Run → Project Runtime. Injects PLAYWRIGHT_HEADLESS into spawned agent env and appends --headless when starting @playwright/mcp.
• Lowers default Claude SDK max_turns for the main agent client from 1000 to 300, with env override AUTOCODER_AGENT_MAX_TURNS.

Notes:

• Default remains headed (no behavior change unless toggle enabled).
• Updated tests for project runtime settings env mapping.

How to verify:

• Enable Playwright headless for a project and start agent; Playwright runs without a visible browser window.
• Optionally set AUTOCODER_AGENT_MAX_TURNS=1000 to restore previous behavior.