Separate passphrase training from Persistent Storage setup #267

persistent-setup mixes storage prerequisites with passphrase capture and training initialization, while spaced-repetition maintains its own prompt/handling path. Keeping these concerns together makes both harder to review and update.

Acceptance:

• Make storage setup responsible only for the Tails storage/feature prerequisites (Persistent Storage setup: make required features explicit and stop stale UI #251).
• Keep training supported through a separate explicit entry point; share the smallest necessary passphrase-prompt adapter using maintained upstream tools.
• Preserve schedule/practice behavior from Option to Restart Spaced Repetition #181/passphrase: Persist repetition schedule #217 and respect existing screen-lock/administration passwords.
• Do not duplicate Tails passphrase-strength policy or persist/log passphrase plaintext; clear transient secrets promptly and handle cancellation/failure explicitly.
• Verify setup and practice, cancellation, rebooted schedule and an existing account password on supported Tails.

Related: #214, #240/#243, #77. Implement against the post-#202 code, rather than preserving removed legacy GTK helpers.