feat: resume recording into an existing dataset across sessions

[ZouzouWP]

Recording a full dataset rarely happens in one sitting. Before this, adding more episodes to an existing dataset later meant starting a brand new one and merging them by hand — nothing stopped you from silently mixing sessions with different FPS, cameras, or task wording into a broken combined dataset.

What changed

• DatasetPicker lists existing local/Hub datasets with their episode count, and offers a "Resume" action next to "Create new".
• Resuming pre-fills FPS, cameras, robot type, and the last task description from the dataset's own metadata (via a new tasks field on /dataset-info), and blocks recording if the current camera/robot setup doesn't match what the dataset was originally recorded with — this is exactly the mismatch that used to fail silently.
• An optional episode-count target (stored in localStorage, not the dataset itself) drives a progress bar across sessions, e.g. "14 / 45 episodes".
• Backend fix: LeRobotDataset.resume() was being called with root=None, which current lerobot rejects. Now resolves the same default local directory LeRobotDataset.create() uses.

sequenceDiagram
    participant U as User
    participant L as Landing page
    participant API as LeLab backend
    participant DS as LeRobotDataset

    U->>L: Click "Resume" on a dataset
    L->>API: POST /dataset-info {repo_id}
    API->>DS: LeRobotDataset(repo_id)
    DS-->>API: fps, cameras, robot_type, tasks
    API-->>L: pre-fill form
    Note over L: Blocks start if current setup mismatches
    U->>L: Confirm, start recording
    L->>API: POST /start-recording {resume: true, repo_id}
    API->>DS: LeRobotDataset.resume(repo_id, root=resolved_root)
    DS-->>API: existing episodes preserved
    API-->>L: new episodes appended after the last one

Loading

Testing

Validated end-to-end on the physical SO-101: episodes appended to a 5-episode dataset across two separate sessions on different days, episode count and video/frame continuity confirmed on the Hub afterward.