Prevent corrupt pairing state from blocking Bluetooth startup

[enaboapps]

Summary

• Add shared atomic JSON write helpers for app-owned state files.
• Recover corrupt pairing state by backing up the unreadable file and creating fresh pairing state.
• Use atomic writes for pairing state, cursor overlay settings, and pointer movement settings.

Closes #257

Validation

• npm test -- src/main/json-file-store.test.ts src/main/pairing/pairing-store.test.ts src/main/cursor-overlay-settings-store.test.ts src/main/pointer-movement-settings-store.test.ts src/main/bluetooth/bluetooth-transport.test.ts
• npm test
• npm run build