TUI wizard shows ephemeral overlay as storage — no persistence partition setup

[eshork]

Symptom

The TUI wizard storage step reports ~8GB available for models. This is the ephemeral overlay (squashfs + tmpfs), not a real persistence partition. On a 500GB USB drive, the remaining ~492GB is unpartitioned and unused. The wizard doesn't offer to create a persistence partition.

Expected Behavior

The wizard should:

1. Detect that no persistence partition exists
2. Show the total unpartitioned space available on the boot USB device
3. Offer to create a persistence partition using that space
4. Clearly distinguish between ephemeral overlay storage and persistent storage

Relationship

Related to #6 (persistence partition auto-creation). This issue is specifically about the TUI wizard's storage step being misleading and non-functional. #6 covers the underlying partition creation capability.

[eshork]

Design Decision (from planning session)

Scope: Full detect + create — auto-detect boot USB, show unpartitioned space, offer to create persistence partition with confirmation.

This is the recommended approach despite the complexity/risk. The wizard should:

1. Detect the boot USB device
2. Show total unpartitioned space available
3. Offer to create the persistence partition with user confirmation
4. Handle partition creation at runtime using existing prepare-usb.sh logic

Safety measures needed: device detection must be bulletproof (only target the boot USB, never internal drives), require explicit user confirmation before any partition operations.

[eshork]

resolved in #15