fix: Clarify shared-secret approval text to prevent misleading consent

ui_display_privacy_shared_secret() told the user they were providing a
"public secret key". The corresponding command path is actually an
X25519 derivation that returns a 32-byte shared secret computed from
the device-held private key and an external public key, not a public
value. On a hardware wallet, approval text is the last security
boundary; inaccurate wording can materially change whether a user
consents to secret disclosure.

Rename the confirmation header to "Provide derived shared secret" so
the title accurately describes the secret material being released.

(cherry picked from commit 7dff261be26af87b7db44935d679d74f64ee32f8)