fix(app): polyfill requestIdleCallback for iOS Safari

[MarkusNeusinger]

Summary

• iPhone users could not use the app: every iOS Safari (and CriOS, which uses WebKit) instance threw TypeError: window.requestIdleCallback is not a function at AppDataProvider mount, which ErrorBoundary caught — leaving the user stuck on the error fallback. Surfaced by the on-device debug console added in feat(debug): on-device console + richer ErrorBoundary for mobile crashes #5808.
• requestIdleCallback is documented as unsupported on Safari/iOS by caniuse, MDN, and WebKit bug 164193. It exists only behind the Experimental Features toggle, which is off by default — so this is a hard regression for every iOS visitor, not a one-off device bug.
• Fix is an inline feature-detect in the existing useEffect: use requestIdleCallback if present, otherwise fall back to setTimeout(cb, 1). Cleanup picks the matching cancel function via captured boolean. No new npm dependency, no global mutation.
• Adds a regression test that stubs both idle APIs as undefined and asserts the three initial fetches still fire and children still render.

Original iPhone error report

Message: window.requestIdleCallback is not a function. (In 'window.requestIdleCallback(async()=>{try{let[e,t,n]=await Promise.all([fetch(`${T}/specs`),fetch(`${T}/libraries`),fetch(`${T}/stats`)]);...},{timeout:2e3})')
URL: https://anyplot.ai/?debug=1
User-Agent: Mozilla/5.0 (iPhone; CPU iPhone OS 26_4_2 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) CriOS/148.0.7778.100 Mobile/15E148 Safari/604.1

Test plan

• cd app && yarn test --run — 467/467 pass, including new falls back to setTimeout when requestIdleCallback is unavailable (iOS Safari) case in Layout.test.tsx.
• cd app && yarn build — TypeScript / Vite build green.
• Pre-existing lint findings on main unchanged; no new findings introduced by this PR.
• Production verification on the reporter's iPhone after deploy (visit https://anyplot.ai/?debug=1, confirm home page loads and the Eruda console is clean).

Why this approach (not a polyfill package)

• Two well-known polyfills exist (aFarkas/requestIdleCallback, pladaria/requestidlecallback-polyfill) but both internally just wrap setTimeout — there is no way to truly polyfill real browser idle time. Adding a dependency would ship bytes to every modern-browser user for the same effect we get inline.
• Single call site in the entire repo — per CLAUDE.md "no abstractions beyond what the task requires", a per-file inline fix is the right shape. If a second call site appears later, that is the moment to extract a utils/idleCallback.ts.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[Copilot]

Pull request overview

Fixes a hard crash on iOS Safari/WebKit where window.requestIdleCallback is undefined, preventing the app from loading by ensuring AppDataProvider schedules its initial data fetches via a safe fallback.

Changes:

• Feature-detect window.requestIdleCallback and fall back to setTimeout when unavailable; clean up with the matching cancel API.
• Add a regression test that simulates iOS Safari by stubbing the idle APIs as unavailable and asserting initial fetches still occur and children render.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
app/src/components/Layout.tsx	Adds requestIdleCallback feature-detect with setTimeout fallback for initial shared-data loading.
app/src/components/Layout.test.tsx	Adds a regression test covering the fallback behavior when idle callbacks are missing.

[MarkusNeusinger]

Addressed in 585ba50 — added vi.unstubAllGlobals() at the end of the test so subsequent tests do not see leaked undefined globals.