fix(collections): prevent sync pull from wiping local collections

[cursor]

Summary

Restores a safety guard in CollectionSyncService.pullFromServer that was accidentally removed in commit 5a0b6237. When the server returns an empty collections payload ([] or null) but the device already has local collections, the app now preserves local data instead of applying the empty remote state.

Bug and impact

Data loss: Users with locally created collections could lose them on the next sync pull if the server row was still empty (first sync, offline usage, or migration). The pull applied [] over non-empty local state, and debounced push could then persist the wipe to the server.

Root cause

5a0b6237 refactored JsonNull handling and dropped this block:

if (remoteJson == "[]" || remoteJson == "null") {
    if (currentCollections.isNotEmpty()) return // preserve local
}

Fix

• Reintroduce the preserve-local guard using isRemoteCollectionsEmpty() (handles JsonNull, empty JsonArray, and legacy serialized forms).
• Add CollectionSyncServiceTest for empty-remote detection.

Validation

• Logic reviewed against pre-regression behavior.
• :composeApp:desktopTest not run in CI agent environment (missing mediamp-mpv dependency on Linux host).