Passphrase UI improvements

When entering a passphrase on the on-screen keyboard, two UX issues make input error-prone:

1. Shift should be one-shot, like on Android. After tapping Shift and then one uppercase letter, the keyboard should switch back to lowercase. Right now it stays in uppercase, so the next letters are often capitalized by accident.

2. Done and Back should ask for confirmation. Accidental taps leave the passphrase screen and discard the input. Several customers have hit this and had to type the whole passphrase again.

Locate the on-screen keyboard and passphrase-screen entry points, then trace how Shift changes keyboard state and how Done and Back leave the screen. Verify that Shift resets after one uppercase character and that both exit actions request confirmation without discarding input unexpectedly.

Written by the indexing model from the issue text.