⚡ Bolt: Prevent duplicate API calls on BlogApp mount

[schmug]

💡 What: Added a module-level cache (fetchPromise and cachedPayload) to BlogApp.tsx for the /api/blog.json fetch request.
🎯 Why: In the windowed OS architecture, BlogApp mounts and unmounts frequently as the window is opened and closed. Previously, every open triggered a redundant network request.
📊 Impact: Reduces redundant network requests for blog posts to exactly 1 per session. Subsequent opens of the BlogApp window are now instant.
🔬 Measurement: Verify by running pnpm run test and pnpm test:e2e. Also verified by code review and passing checks. Checked that there are no regressions.

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PR created automatically by Jules for task 17233764467552576775 started by @schmug

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