fix(installer): never expose a fake disk as a real target

[amandeavor]

Problem

velocityinstall/src-tauri/src/partition.rs::get_disks returns a realistic fake /dev/sda disk when /sys/block is unavailable. A fake device should never appear as a real installation target, even on an unsupported host.

Proposed direction

Return a clear unsupported-platform error in production. If UI development needs sample disks, move them behind an explicit mock or demo mode that cannot be enabled accidentally in a release build.

Acceptance criteria

• unsupported hosts never receive a plausible real-device path by default
• demo data is explicitly opted into and visibly marked as simulated
• the UI handles the unsupported result without offering installation
• the behavior is covered by a focused test where practical
• component status documentation is updated

Treat this as safety-sensitive. Do not test partition commands against a real disk.