Dashboard /runs adaptive auto-refresh

The runs browser and agent-detail page in the dashboard now refresh automatically using an adaptive cadence.

Behavior:

• 3-second refetch interval when any visible run has status === 'running'
• 15-second interval when all visible runs are terminal
• Paused entirely when the tab is in the background (no wasted requests)

Both pages inherit the behavior because they consume the same useRuns hook. No API changes — the dashboard just stops asking you to F5.

Impact: active runs surface as they complete. Analytics dashboards and run-detail pages get the same treatment in a follow-up.