Add mergeSession() function that loads another session's manifest and appends its items to the current view. Items are tagged with _session field and deduplicated by ID. Each session row in the picker gets a "+Merge" button. The merge is view-only (not saved) with a notification toast showing how many images were added. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>