fix: misleading disk space indicator for shared ZFS pools (#674)

Closes #674 (closed)

When multiple datasets share a single ZFS pool, the disk usage progress bar shows a misleading visualization. The progress bar fill is calculated as used / size (dataset-specific used / total pool size).

For example, it might show only ~31% filled for a dataset using 8.9 GiB of a 28.5 GiB pool (implying 69% is free). In reality, only 597 MiB (2%) of the pool is actually free because other datasets consume the rest.

The text labels in this case ("Used: 8.899 GiB / 31%", "Free: 597.064 MiB / 2%") are individually correct but the progress bar contradicts the "Free: 2%" label by appearing mostly empty.