⚡ Bolt: [performance improvement] React.memo list optimization

[primoscope]

💡 What

• Extracted DownloadItem and LogItem into separate components and wrapped them in React.memo().
• Added a custom equality function for DownloadItem to only re-render when visible primitive fields change.
• Replaced array index keys with stable generated IDs for LogItem.
• Moved static helper functions out of the component bodies to avoid recreation on every render.
• Added explanatory comments for the performance optimizations.

🎯 Why

• ProgressPanel: The ProgressPanel mapped over active downloads directly in its render block. During high-frequency WebSocket updates (which are common during fast downloads), the entire list of downloads would re-render for every single progress tick.
• LogPanel: Similarly, the LogPanel mapped over logs and used array indices as keys. Appending a single new log message via WebSocket triggered a re-render of the entire historical log list.

📊 Impact

• Substantially reduces React re-renders during high-frequency WebSocket updates (progress ticks, new logs).
• LogPanel will only render the new log item being appended rather than the entire history.
• ProgressPanel will only re-render the specific download item that received a progress or status update, leaving the rest of the active downloads list untouched.

🔬 Measurement

1. Open the UI and start a download with many files/chunks.
2. Open the React DevTools Profiler and record a session while the download is active.
3. Observe that only the specific DownloadItem component re-renders when its progress changes, instead of the entire ProgressPanel and all its children.
4. The same applies for incoming log messages in the LogPanel.

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PR created automatically by Jules for task 6500790666116220388 started by @dzp5103

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