🧹 Extract FolderHeaderRow from FolderView to improve readability

[Matuyuhi]

🎯 What: The FolderView component was over 300 lines long and highly complex due to a massive <FolderHeaderBox> block. I extracted this header block into its own dedicated FolderHeaderRow component within the same file.
💡 Why: Functions and components over 150 lines typically have multiple responsibilities. Extracting the header improves maintainability, isolates header-specific state (useEditMode, syncing, menuId), and simplifies the main FolderView rendering path.
✅ Verification: Verified with bun run tsc --noEmit to ensure type safety, and bun run test to guarantee no behavior regressions. The code review confirmed it's clean and safe.
✨ Result: A more readable, modular FolderView component without any behavioral changes.

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PR created automatically by Jules for task 12445313845800389472 started by @Matuyuhi

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