fix: show actual dates instead of Open/Closed in PR Date column (#1111)

[alpurkan17]

Summary

The PR table Date column rendered literal strings Open/Closed for non-merged PRs instead of actual dates, duplicating status info already shown in the Status column and tabs while hiding the real date.

Fix: use pr.closedAt || pr.prCreatedAt fallback instead of hardcoded Open/Closed strings in MinerPRsTable and search PullRequestsTab.

Validation

• npm run build — no regressions
• Merged PRs still show mergedAt
• Closed PRs show closedAt (or prCreatedAt fallback)
• Open PRs show prCreatedAt

Closes #1111

[alpurkan17]

@anderdc @LandynDev ready for review

[anderdc]

PRs must target the test branch, not main — see CONTRIBUTING.md. Closing.