Include rank check date in Rank Tracking exports

[bensenescu]

Problem

Rank Tracking exports omit when the rankings were checked. That makes saved CSV and Sheets files harder to interpret or compare later.

Evidence

• User feedback: “Include the date of the rank tracking in the export.”

Implementation check

Checked main at fec642a0:

• src/client/features/rank-tracking/RankTrackingTableParts.tsx builds CSV/Sheets rows without a date column.
• src/client/features/rank-tracking/RankTrackingDomainDetail.tsx already receives the latest run’s lastCheckedAt value.
• src/server/features/rank-tracking/services/rankTrackingResults.ts returns the stored check timestamp.

The timestamp already exists; the export path only needs to carry and format it.

Suggested direction

Add one machine-sortable Last checked at column to both full-table and selected-row CSV/Sheets exports.

Contributor fit

Good first issue: small, localized, and testable without changing data collection.

Acceptance criteria

• Full-table and selected-row CSV exports include Last checked at.
• Sheets exports include the same field and value.
• The format is consistent and sortable in common spreadsheet tools.
• Existing export columns and filtered-row behavior remain unchanged.

[mariazuheros]

Hi! I'd like to take this one — I've got a PR ready that adds a machine-sortable Last checked at column to both the full-table and selected-row CSV/Sheets exports. Opening it now.