Add user PIN protection (#719)

## Description
Adds (back) the ability to set a per-user PIN. This is required to
switch to the user. If the user has a PIN, this overrides and ignores
the "Sign in automatically" setting for that user.

The PIN consists of any combination of D-Pad directional buttons. If you
forget the PIN, you can click the button to login via the server instead
(ie QuickConnect or username/password) which will also remove the PIN so
you can set a new one if desired.

### Related issues
Closes #268 
Replaces #398
Related to #321 & #356

### Screenshots
![enter_pin
Large](https://github.com/user-attachments/assets/aa25364d-1266-410d-846e-8dc32c56a794)

app/src/main/java/com/github/damontecres/wholphin/data/ServerRepository.kt

@@ -142,8 +142,6 @@ class ServerRepository
                 } else {
                     val user = serverAndUsers.users.firstOrNull { it.id == userId }
                     if (user != null) {
-                        // TODO pin-related
-//                if (user != null && !user.hasPin) {
                         return changeUser(serverAndUsers.server, user)
                     }
                 }