Settings: change the login's password, or delete the login

A Login section in Settings changes the root's password (the current one
given) and deletes the whole login, every persona with it, after its
password and an explicit "I understand"; the client then signs out. Both
use PrivaPub's existing /clientapi/user routes.

The e2e signs up a login of its own, changes its password and deletes it,
checking each against PrivaPub's login. Signing in now waits for either
the feed or an enabled persona chooser, so a login with one persona (which
goes on by itself) and a new persona's empty feed both sign in.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

ClientJsonContext.cs

@@ -32,6 +32,8 @@ namespace decePubClient;
 [JsonSerializable(typeof(ValidationErrors))]
 [JsonSerializable(typeof(WebResult))]
 [JsonSerializable(typeof(LoginForm))]
+[JsonSerializable(typeof(UserPasswordForm))]
+[JsonSerializable(typeof(RemoveSelfForm))]
 [JsonSerializable(typeof(JwtUser))]
 [JsonSerializable(typeof(ViewAvatar))]
 [JsonSerializable(typeof(List<ViewAvatar>))]