Carry next through the password change and let requesters pick a username

A forced or voluntary password change now redirects to the validated
`next` target (the page the user was heading for, or the home page)
instead of landing back on the account form. Login and the
must-change-password middleware pass the destination along as
/account?change=1&next=... and the form carries it as a hidden field.

The request-access form gains a required Username field validated with
the account rules and rejected when an account or another open request
already holds it (case-insensitive). Migration 0009 stores it on the
request; the notification email and the Users dashboard show it, and
the Approve form is prefilled with it.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QVPagF6jfDv78CC5Jv2wp4

migrations/0009_request_username.sql

@@ -0,0 +1 @@
+ALTER TABLE account_requests ADD COLUMN username TEXT;