Batch 11 — Settings and the App Lock
A user reported that Settings → App lock could not be left. Tracing it found three causes (the bottom-nav back-stack restore pins the Settings tab to its child; no settings page has a back arrow; setting a PIN can race the gate and lock you out of the session you set it in), a real security bug beside them (Change PIN never checks the current PIN), and a lock model that offers biometrics only as a shortcut on a mandatory PIN.
This batch gives every settings page a way back, makes the Settings tab always land on its root, fixes the bypass and the race, lets the user choose a PIN just for this app, fingerprint or face, or either — with copy that says plainly the PIN is the app's and not the phone's — and brings the lock's error handling, tests and security docs up to what ships.
Landed when: every issue below is closed with its named test or device-row evidence; a settings sub-page cannot be entered without a back arrow; SECURITY.md describes the lock that ships and no phantom biometric key.
