Fix clippy unnecessary_unwrap warnings and rustfmt formatting

[Copilot]

Three instances of the is_some() + .unwrap() anti-pattern were triggering clippy::unnecessary_unwrap. A follow-up cargo fmt failure was also introduced by the refactor.

• src/interface.rs: command.last_run.is_some() + .unwrap() → if let Some(last_run) = command.last_run; collapse .signed_duration_since(...) per rustfmt
• src/stats_generator.rs: settings.stats_only_dir.is_some() + .as_ref().unwrap() → if let Some(stats_only_dir) = &settings.stats_only_dir
• src/main.rs: settings.append_to_histfile.is_some() + .as_ref().unwrap() → if let Some(append_to_histfile) = &settings.append_to_histfile

// Before
if settings.append_to_histfile.is_some() {
    let histfile = PathBuf::from(settings.append_to_histfile.as_ref().unwrap());

// After
if let Some(append_to_histfile) = &settings.append_to_histfile {
    let histfile = PathBuf::from(append_to_histfile);

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