Build character relationship controls

[jing2003]

Acceptance criteria

• Display a character's relationships on the character profile page.
• Clearly show the related character, relationship type, direction, and description.
• Allow users to connect the current character to another character in the same project.
• Allow users to edit the relationship type or description.
• Allow users to remove a relationship without leaving the profile page.
• Prevent the current character and already-connected options from appearing as invalid choices.
• Link each related character to their character profile.
• Display relationship validation and API errors clearly.