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 <noreply@anthropic.com>
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