⚡ Bolt: [performance improvement]

[juntaochi]

💡 What: Replace Paragraph::new(" ".repeat(width)) with Block::default() for rendering background scanlines in draw_screen_border.
🎯 Why: Using " ".repeat allocates a new String on the heap for every single scanline, on every single render frame, which is inefficient. Using an empty Block achieves the exact same visual background fill but requires zero heap allocations.
📊 Impact: Eliminates H/2 (where H is terminal height) string heap allocations per render frame when the screen border is drawn, improving TUI render efficiency.
🔬 Measurement: Code inspection confirms Block::default() combined with a background style fills the Rect area in Ratatui without needing any text or string allocation. Benchmark confirms generating a new String::with_capacity and filling it with spaces per iteration is much slower than returning a static or zero-allocation background property.

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PR created automatically by Jules for task 13051093788142648032 started by @juntaochi

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