Add Last-Update Timestamp Display

[w2naf]

Description

Add a visible timestamp showing when the dashboard data was last refreshed. This helps operators know how current the displayed propagation data is.

Acceptance Criteria

• Timestamp displayed in the UI (e.g., header or status bar)
• Updates each time new data is fetched from the API
• Format is clear and readable (e.g., "Last updated: 14:30:15 UTC")
• Works for both map and table views

Blocked By

• Install, Test, and Debug Current DXDashboard on W3USR System #31 (Install, Test, and Debug Current DXDashboard)

Milestone

Milestone 1: Minimal Working System (Feb 28)