Description

Users currently have no way to know how old the data in a panel is. Add a relative timestamp ("2m ago", "just now", "12m ago") to each DashPanel's border_title showing when its source's snapshot was last refreshed.

Implementation:

• Extract refreshedAt from each source's snapshot during refresh_data()
• Pass it to DashPanel alongside the content (e.g. set_content(text, refreshed_at=dt))
• DashPanel updates border_title = f"{self._title} {relative_time(refreshed_at)}"
• Add a lightweight 60s timer that re-renders titles in place (just the relative time strings) without re-fetching data
• relative_time(dt) → "just now" (<2m), "Xm ago" (<1h), "Xh ago" (≥1h)

Acceptance Criteria

• #1 Each panel border shows relative age of its data
• #2 Timestamps update every minute without re-fetching
• #3 Panels without a snapshot (not configured) show no timestamp