Resume CloudKit sync from persisted state instead of full re-fetch

Previously CKSyncEngine was created with nil state serialization on
every launch, replaying the entire zone change history (hundreds of
merge/delete log lines and re-downloads each start). Now the saved
state is loaded so launches only fetch changes since the last session.

To make incremental sync safe:
- Clear persisted state once when the record schema version increases,
  so records previously skipped as unsupported get re-delivered after
  an app update.
- Handle remote zone deletion in fetchedDatabaseChanges by recreating
  the zone and re-uploading local data, instead of leaving sync dead.

Account change and manual Refresh Sync still clear the state and force
a full fetch as before.

Yattee/Services/CloudKit/CloudKitRecordMapper.swift

@@ -22,7 +22,8 @@ final class CloudKitRecordMapper {
 
     /// Current schema version for CloudKit records.
     /// Increment this when record structure changes.
-    private let currentSchemaVersion: Int64 = 2
+    static let currentSchemaVersion: Int64 = 2
+    private var currentSchemaVersion: Int64 { Self.currentSchemaVersion }
 
     init(zone: CKRecordZone) {
         self.zone = zone