Remove dead `parent_key` field from `FlatRow::GroupChild` — suppressed with `#[allow(dead_code)]`

[ooloth]

Current state

FlatRow::GroupChild in ui/tui/src/display.rs carries a parent_key: GroupKey field that is populated during construction but never read anywhere in the codebase. The dead field is kept in place by a #[allow(dead_code)] attribute at line 40, suppressing the compiler warning rather than addressing the underlying issue. This prevents the compiler from enforcing the dead-code invariant going forward and leaves structural noise in the display representation.

Ideal state

• FlatRow::GroupChild does not carry parent_key.
• The #[allow(dead_code)] attribute is absent from the struct.
• The compiler enforces dead-code warnings on display.rs without any suppression.

Starting points

• ui/tui/src/display.rs line 40 — the #[allow(dead_code)] attribute and parent_key: GroupKey field definition

QA plan

1. Open ui/tui/src/display.rs — expect no #[allow(dead_code)] attribute and no parent_key field in FlatRow::GroupChild.
2. Run cargo check from the hub/ root — expect a clean compile with no dead-code warnings for display.rs.
3. Run just test — expect all tests to pass.

Done when

FlatRow::GroupChild compiles cleanly without any #[allow(dead_code)] suppression and parent_key is absent from the struct definition.