fix(save-dialog): allow navigating to other drives (#170)

The custom Save As dialog navigated up the tree solely via
`Path::parent()`, which returns `None` at a drive root (`C:\`).
Navigation therefore dead-ended on the starting volume, so other
drives and USB sticks were unreachable.

Add a synthetic Windows "This PC" drives root above the drive
letters. `parent_folder()` now climbs `…\sub → C:\ → This PC`, and
`read_dir_entries()` lists the available volumes (probed A:–Z: by
stat, no new dependency) when at that root. The dialog's up-button
and path bar route through the same helper and show "This PC".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/view.rs

@@ -3471,8 +3471,12 @@ fn save_as_dialog_window<'a>(
     };
 
     // ── Path bar ─────────────────────────────────────────────────────────
-    let path_str = folder.to_string_lossy().into_owned();
-    let up_path = folder.parent().map(|p| p.to_path_buf());
+    let path_str = if crate::io::is_drives_root(folder) {
+        "This PC".to_string()
+    } else {
+        folder.to_string_lossy().into_owned()
+    };
+    let up_path = crate::io::parent_folder(folder);
     let path_bar = row![
         {
             let up_msg = up_path.map(Message::SaveDialogNavigate);