feat: add SAM3 support in AI-Box mode with multi-shape output

[wkentaro]

• Add SAM3 to the AI model list in the AI-Assisted Annotation widget
• Refactor AI inference to return shapes instead of mutating in-place (_shapes_from_points_ai, _shapes_from_bbox_ai, _shapes_from_ai_response)
• Support multiple shapes from a single AI bbox annotation (e.g., SAM3 detecting multiple objects)
• Label dialog applies to all shapes from one annotation; undo removes all unlabeled shapes at once