Preserve new snippet drafts when creation fails

[hackjutsu]

Summary

• save each submitted new snippet as a per-user local draft before the GitHub request
• keep the editor open on request failure and restore the draft after an app restart
• clear the draft only after successful creation and explain recovery in localized notifications

Testing

• npm run lint
• npm test (216 tests and webpack development build)
• npm run test:smoke (Electron app launch plus authenticated render fixtures, including the New editor)

Live GitHub Gist creation was not exercised against a real account; failure and recovery behavior is covered by focused unit tests.

Closes #414