Build scene-character assignment controls

[jing2003]

Acceptance criteria

• Display the characters currently assigned to a scene.
• Allow users to select and assign an existing character from the same project.
• Allow users to enter or edit role_in_scene.
• Allow users to enter or edit knowledge_gained.
• Allow users to remove a character assignment without navigating away.
• Prevent already-assigned characters from being assigned again.
• Display assignment errors clearly.