perf: defer service worker registration with requestIdleCallback (#461)

[gbengaeben]

Summary

Defers service worker registration using requestIdleCallback to avoid blocking the initial paint and improve Time to Interactive (TTI).

Problem

ServiceWorkerRegistration was eagerly registering the service worker on every page load (via load event or document.readyState === 'complete'), including on routes that don't require offline support. This blocks the main thread and increases TTI.

Solution

• Added scheduleWhenIdle() helper that uses requestIdleCallback with a 5-second timeout
• Falls back to setTimeout(2000) when requestIdleCallback is unavailable
• SW registration is now deferred until the browser is idle
• startQueueAutoFlush() is still called eagerly (it's lightweight)
• Properly cancels idle callback on component unmount

Changes

• ServiceWorkerRegistration.tsx: Added scheduleWhenIdle helper, removed eager register-on-load logic
• ServiceWorkerRegistration.test.tsx: Rewritten from vitest to jest, added tests for deferred registration, idle callback, fallback to setTimeout, cancellation on unmount, and error handling (10 tests passing)

Closes #461

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[nanaf6203-bit]

Smart perf fix. Merging.