iCloud Sync: per-record ID-based merge (currently last-write-wins)

[wsclx]

Symptom

Current iCloud Sync (shipped in 0.7.0-beta) is last-write-wins by file mtime for the entire automations.json. If two machines edit different automations simultaneously, one side's edits are silently overwritten.

Acceptance

• Per-automation merge by id field.
• Conflict detection (same id, different updated_at on both sides) → user-visible diff with manual resolution.
• Tombstones for deleted records so a deletion on machine A doesn't get resurrected by an old copy on machine B.

Scope

0.8.x. Documented in CHANGELOG [Unreleased] Planned for 0.8.x.

References

• Current implementation: src-tauri/src/icloud_sync.rs
• Tauri commands: icloud_sync_status, icloud_sync_push, icloud_sync_pull in src-tauri/src/lib.rs

Future scope

• Also sync custom-LED paint snapshots (currently local-only).
• Also sync theme/setting preferences.