feat: make stats polling visibility aware

[gaoflow]

Summary

• pause stats polling while document.visibilityState is hidden
• resume with an immediate fetch when the tab becomes visible again
• add a manual Refresh button and Last updated indicator
• keep the last good stats visible after an error and stop interval retries
• replace inline tiles with shared StatTile

Closes #10

Tests

• npm test -- src/app/stats/page.test.tsx --runInBand
• npm run lint
• npm test -- --runInBand
• npm run build
• NEXT_PUBLIC_STABLEROUTE_API_BASE=http://localhost:3011 npm run build
• Rendered smoke: started a temporary mock stats API on 3011 and production Next server on 3010, then captured /stats with Playwright CLI.

Browser runtime fallback: the in-app Browser was available as a skill, but the runtime returned Browser is not available: iab; I used Playwright CLI screenshots instead.

Screenshot evidence saved locally:

• /tmp/stableroute-stats-visibility-polling.png

[mikewheeleer]

thanks for jumping in @gaoflow! 🙏 we assign issues before work starts so two people don't build the same thing, and #10 isn't assigned to you. please grab an open unassigned issue and PR against that. closing this one for now, hope to see you back 💪