fix: restore backup list scrolling

[OneNoted]

Summary

• apply backup list scroll-window logic so the selected row stays visible
• reset backup list scroll state when filters empty the view or the viewport has no visible rows
• keep delete actions aligned with the visible selection in long filtered lists

Verification

• cd tui && zig build test -Doptimize=ReleaseSafe
• cd tui && zig build -Doptimize=ReleaseSafe

[chatgpt-codex-connector]

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