Add merge-based conflict resolution (instead of LWW)

[limeflash]

What

Currently sync uses last-writer-wins per workspace UUID. For a better UX, implement merge:

• Tabs identified by zenSyncId (stable, provided by Zen)
• Detect tab additions/removals across devices
• Merge workspace membership (union of tabs)
• Detect conflicts (same tab moved to different workspace on 2 devices → pick latest)

Approach

Use vector clocks or CRDT-style merge. The relay stores per-device snapshots; merge happens client-side.