fix: non-blocking browser launch in Pi extension (Linux)

[backnotprop]

Summary

• Replace execSync(xdg-open ...) with spawn(..., { detached: true, stdio: "ignore" }).unref() in the Pi extension's openBrowser()
• execSync blocks the Node.js event loop, so the HTTP server can't respond to the browser's initial page request on Linux desktops where xdg-open doesn't immediately exit — causing a deadlock

Closes #288

Test plan

• Test on Linux (Wayland/Hyprland) with Chromium — verify page loads without hanging
• Test on macOS — verify open still works
• Test with PLANNOTATOR_BROWSER override

🤖 Generated with Claude Code

[backnotprop]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

Reviewed the execSync → spawn().unref() refactor in apps/pi-extension/server.ts. The fix correctly addresses the deadlock described in #288 where xdg-open blocks the Node.js event loop on Linux. All platform branches (macOS, Windows/WSL, Linux) and custom browser env var handling are preserved. Control flow is exhaustive and compiles cleanly under strict mode.

🤖 Generated with Claude Code

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