Replace module-level refs and circular dependencies with a centralized
mention state hook (useMentionState) and shared types (mention-types).
- Introduce mention-types.ts for shared types (avoid circular deps)
- Introduce mention-state.ts with useMentionState hook (replace global refs)
- Rewrite MentionInput as contenteditable with AST-based mention rendering
- Rewrite MentionPopover to use props-only communication (no module refs)
- Fix MessageMentions import cycle (mention-types instead of MentionPopover)
- Wire ChatInputArea to useMentionState with proper cursor handling