feat(save-dialog): extend drive navigation to macOS and Linux

Issue #170's fix added a "This PC" volumes root above the drive
letters on Windows. Generalize it so the Save As dialog can switch
volumes on every platform:

- parent_folder() now climbs the filesystem root into the volumes
  list on all platforms (`/` -> Computer), not just Windows.
- available_drives() is platform-specific: Windows probes A:-Z:,
  macOS lists `/` + /Volumes/*, Linux lists `/` + the removable
  mounts under /run/media/$USER, /media/$USER and /mnt. Other Unix
  and wasm fall back to `/`.
- drives_root_label() shows "This PC" on Windows, "Computer" elsewhere.
- read_dir_entries() lists the volumes at the drives root on every
  platform.

Also gitignore the dist/ web build output so the wasm binary is never
committed by accident.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/view.rs

@@ -3472,7 +3472,7 @@ fn save_as_dialog_window<'a>(
 
     // ── Path bar ─────────────────────────────────────────────────────────
     let path_str = if crate::io::is_drives_root(folder) {
-        "This PC".to_string()
+        crate::io::drives_root_label().to_string()
     } else {
         folder.to_string_lossy().into_owned()
     };