⚡ Bolt: Preload search index in background

[saint2706]

⚡ Bolt: Preload search index in background

Background indexing is now triggered in App.tsx using a useEffect. This starts caching documents shortly after the initial mount, improving the time to first search results (reducing interaction latency/INP) without blocking the main UI thread, thanks to our requestIdleCallback implementation in searchIndex.ts.

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PR created automatically by Jules for task 3361699043829957768 started by @saint2706

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