Harden account and auth persistence with atomic writes

[thatdaveguy1]

Why

The account database and active auth file are written directly today. If the app crashes mid-write or multiple operations overlap, that can leave accounts.json or auth.json in a partially written state.

This PR hardens those persistence paths with atomic writes, simple locking, and backup files so storage updates are more crash-resistant.

What changed

• add shared filesystem helpers in fs_utils.rs for locked atomic writes
• update storage.rs to use the new write path for account persistence
• update switcher.rs to write active auth data atomically and emit .bak backups
• ensure lockfile parent directories are created for first-run paths

Reviewer notes

• this PR is intentionally limited to persistence safety
• backup security mode selection and process restart behavior are handled in separate PRs

Verification

• cargo check in src-tauri passes on this branch

[Lampese]

plz resolve the conflicts