Add cross-platform user preferences and gear storage

[exula]

Summary

• add first-run storage-folder onboarding with platform-specific defaults
• remember the selected library in atomic, versioned user preferences
• add in-app open/create and copy-and-switch library controls
• preserve one-launch-only behavior for explicit --data paths
• expand tests and CI coverage across Windows, macOS, and Linux

Test plan

• uv run --isolated --python C:\Users\Admin\.cache\codex-runtimes\codex-primary-runtime\dependencies\python\python.exe --frozen python -m unittest discover -s tests -v
• uv lock --check

All 27 tests pass.