Auto-refresh cadence ring (#33)

Motivation

The RefreshRing component existed but was unused. The Auto-refresh chip let users pick a refetch interval but gave no visual sense of the rhythm.

User-facing change

• The Auto-refresh chip now shows a small countdown ring next to the interval dropdown, ticking down to the next refresh cycle and resetting when the interval changes. The countdown pauses while the tab is hidden.

Honesty note: dashboard cards refetch on their own TanStack Query timers, so the ring reflects the configured cadence, not a guarantee tied to a single query's refetch. It is a rhythm indicator, which is why the title stays generic.

Code change summary

• web/src/hooks/useAutoRefresh.tsx: added a secondsToRefresh countdown (1s ticker, resets on interval change, pauses when document.hidden) to the context.
• web/src/pages/Dashboard/AutoRefreshChip.tsx: render RefreshRing with secondsToRefresh + total.

Validation evidence

• cd web && npm run build → clean.
• npx eslint on both changed files → clean.

No backend / API / IaC changes.