Display private key after initialization

The Jade doesn't allow for displaying the private key after initialization. Use cases:

- BIP39 words:
Confirm the backup words list with the device words list
Create a backup (if not done initially or lost)
- SeedQR:
Create SeedQR backup from an existing private key if not done initially
Use SeedQR as input for SeedHammer

Consider displaying a security warning that when following these steps anyone can steal funds when viewing the output.

(Similar to 'View Seed Words' for Coldcard MK4 and Foundation Passport)