Allow powering off pool-member disks with strong confirmation for hot swap #113

The Power off button is currently hidden for disks that are members of a pool (and for mounted disks), so it is impossible to power down a bay disk right before pulling it out to hot-swap it.

Requested behavior:

• Show the Power off button on every disk row.
• For pool-member disks: require a written confirmation (type the pool name or the disk name) with a clear warning that the pool will go DEGRADED until the disk is replaced or the pool resilvers.
• Keep the current double-click confirm for free disks; keep the mounted/swap veto (never power off a disk with active mounts).