Show recent files in the Open dialog

[kisaraki]

Summary

• add a lower pane to Open that lists the five most recently opened absolute paths
• make every recent entry directly clickable
• include successful dialog, command-line, and shell opens in the same MRU list
• persist unique normalized paths in the existing settings file without changing Save As layout

Validation

• unit tests cover newest-first uniqueness, the five-item limit, and LF-only settings output
• cargo test --all-features --all-targets on Windows and WSL/Ubuntu
• cargo clippy --workspace --all-features --all-targets -- --no-deps --deny warnings on Windows and WSL/Ubuntu

Contribution ownership

This contribution is entirely my own work. It is not created on behalf of, or connected with, any organization or company. I agree to contribute it under the repository's MIT License and to the repository's applicable contribution terms.