fix(tui): add project detail section separators

[HonestMajority]

Summary

• Render project-detail AGENTS/TASKS spacer rows as dark gray section separators.
• Keep section labels standalone and label-only.
• Leave normal section/task group spacer rows blank.

Validation

• cargo test project_section
• cargo test project_detail
• cargo fmt -- --check
• cargo clippy --all-targets -- -D warnings
• cargo test
• git diff --check
• tmux TUI smoke at 120x40 and 80x24