fix(pages): keep previous page visible during lazy route transitions

[shaurya703]

Fixes #788

Problem

Navigating to a lazy-loaded page (/benchmark, /quickstart, /docs, /blog) re-triggered the <Suspense> fallback — a plain black <div> — so the whole page went black while the route's chunk downloaded.

Approach

Approach 3 from the issue: keep the existing fallback for the true first-load case, but prevent it from showing during in-app transitions.

• New useTransitionedLocation() hook: returns a location that trails useLocation() by one startTransition. <Routes location={...}> renders from it, so when the next route suspends, React keeps the current page visible (transition semantics) instead of unmounting to the fallback.
• First paint is unchanged — there is no previous content to keep, so the dark fallback still shows, preserving the intentional no-white-flash initial load.
• ScrollToTop now keys off the displayed location, so the scroll reset happens when the new page actually appears rather than while the old page is still on screen.
• No router migration needed — works with the existing <Routes> API on React Router 6.8.

Acceptance criteria

• No black flash during navigation — covered by a test: navigate to a lazy route whose chunk is deliberately unresolved, assert the old page is still on screen and the fallback is absent
• First-time load still shows the dark loading state — covered by a test rendering directly at an unresolved lazy route
• Back/forward navigation — covered by a test using router history
• npm test — 19/19 pass (5 files, including the new one)
• npm run lint — 0 errors (the 2 pre-existing warnings in HeroSection.tsx/MarkdownRenderer.tsx are untouched); tsc --noEmit clean

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[github-actions]

✅ OpenCodeReview: Review complete: 0 finding(s) across 2 selected item(s).

[lizhengfeng101]

LGTM