feature/GH-23-ratatui-multi-pane-layout-shell
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Closes #23. Implements AppLayout 3-pane split (sidebar 20%, list 35%, reader 45%) and ActivePane focus management with 100% unit test coverage.
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Closes #23. Implements AppLayout 3-pane split (sidebar 20%, list 35%, reader 45%) and ActivePane focus management with 100% unit test coverage.