Show entry/exit timestamps on GridLadderCard and add unit tests

[carkod]

Motivation

• Surface ladder timing information in the UI by showing the entry timestamp and, for closed ladders, the exit timestamp so users can see when a ladder started and finished.

Description

• Import the GridLadderStatus value and formatTimestamp util and update the GridLadderCard to display an Entry time row using ladder.created_at formatted via formatTimestamp.
• Add a conditional Exit time row that renders when ladder.status === GridLadderStatus.CLOSED and ladder.closed_at is present, using formatTimestamp for display.
• Adjust imports to bring GridLadderStatus in as a runtime value instead of a type-only import.
• Add new tests GridLadderCard.test.tsx that verify the entry timestamp is shown and that the exit timestamp appears before the Total realized row when the ladder is closed.

Testing

• Ran unit tests with vitest for the component tests in GridLadderCard.test.tsx which include two cases and both tests passed.
• The tests checked for the presence of the Entry time label and formatted entry time, and for the formatted Exit time and its ordering relative to Total realized, and they succeeded.

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Codex Task