fix: make launcher resilient to pywebview startup failures

[LaplaceYoung]

Summary

• Delay pywebview import until native desktop launch, so CLI help and browser mode stay available when pywebview/pythonnet startup fails.
• Set PYTHONNET_RUNTIME=coreclr on Windows through setdefault, preserving explicit user runtime settings.
• Add --browser plus automatic browser fallback when native window startup raises.

Verification

• python -m py_compile launch.pyw ga.py llmcore.py agent_loop.py
• python launch.pyw --help
• mocked open_browser_mode fallback check
• git diff --check

Fixes #18
Refs #136