fix(core/disk): skip unmountable partitions when listing backup targets

[MattDHill]

Summary

• Tiny non-data partitions on a backup disk (GPT header, ESP, etc.) were appearing as "Available for backup, 0 GB" rows alongside the real data partition on the same physical disk. The backup-target list flattens each disk into one row per partition, with no filter for whether the partition is actually usable.
• part_info now returns Option<PartitionInfo>, returning None when the read-only mount probe fails; list() filters those out.
• The OS-disk LVM-PV path in list() is restructured to compute capacity directly via get_capacity rather than going through part_info — raw PVs can't mount anyway, and we only needed logicalname + capacity on that branch. This keeps the existing behavior for the embassy data disk.
• i18n key disk.util.could-not-collect-usage-info is renamed/reworded to disk.util.skipping-unmountable-partition to reflect the new "skip" semantic, across all 5 locales.

🤖 Generated with Claude Code

[dr-bonez]

Just because we know the capacity of an LVM PV doesn't mean it has a mountable filesystem on it

[MattDHill]

Good catch. The previous patch required every partition in the external-disk branch to mount before being added to disk.partitions, which dropped raw LVM PVs (no mountable filesystem on them) — breaking the os_install/setup scan for EMBASSY_/STARTOS_ GUIDs.

Pushed 02c4154ec: pulled the LVM-PV logic from both branches of list() into one lvm_pv_part_info helper that builds a PartitionInfo from capacity + guid + probed LV filesystem without going through the mount probe. The non-PV path still requires mount, which is what filters the tiny header/ESP partitions on the user's Samsung drive.