feat: add last synced timestamp to dashboard header (#186)

[Tannuu18]

Summary

Brief description of what this PR does.

Adds a global dashboard sync timestamp in the header so users can see how fresh their metrics are.

Closes #186

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup

Changes Made

• Added a dashboard-scoped sync context/provider to track the last successful widget fetch.
• Updated the DashboardHeader to show a muted “Synced just now” / “Synced X min ago” label.
• Added a 60-second interval so the displayed timestamp stays current.
• Kept the timestamp hidden until the first successful sync happens.

How to Test

Steps for the reviewer to verify this works:

1. Open the dashboard.
2. Wait for any widget to finish loading data.
3. Confirm the header shows a small muted sync timestamp.
4. Wait a minute and confirm the timestamp increments as expected.
5. Refresh the page and confirm the sync line stays hidden until the first fetch completes.

Screenshots (if UI change)

Checklist

• Linked issue in summary
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Self-reviewed the diff
• Added/updated tests if applicable

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[github-actions]

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