Expose WebView2 CDP on dev launcher

[bindreams]

Closes #129.

Summary

• `scripts/dev.py` injects `WEBVIEW2_ADDITIONAL_BROWSER_ARGUMENTS=--remote-debugging-port=9222` into the GUI process env. Appends to any existing value rather than overwriting.
• Prints platform-specific guidance:
  • Windows: `WebView2 remote debugging: http://127.0.0.1:9222\` — attach via Playwright Chromium driver, chrome-devtools-mcp, or `chrome://inspect`.
  • macOS: `WKWebView remote debugging: Safari → Develop → Hole → Hole Dashboard` — WKWebView ignores the env var (no CDP equivalent), but Safari's Web Inspector reaches it via XPC. CDP-based tools cannot attach; Playwright's WebKit driver is the alternative.

Test plan

• Manual: `uv run scripts/dev.py` and verify the remote debugging URL is printed near startup
• Manual: `curl -X GET http://127.0.0.1:9222/json\` returns the WebView2 page list