refactor(sync): remove syncSource from unified sync module and update docs
Replace syncSource-based push filtering with syncedAt timestamp checks
across the unified sync module. Push filter now uses syncedAt === 0
(never synced) instead of syncSource === ''. Also fixes pre-existing
bugs: _mergeServerItem now converts server ISO timestamps via
fromISOString(), and test data uses correct camelCase field names.
Updated all related documentation to reflect syncSource removal status.