Split Admin module into Status and User Management under one sidemenu dropdown

Splits fn=admin into two independent modules: the original status
dashboard stays at fn=admin, user management moves to a new fn=usr
(IntranetController.AdminUsers.cs, fis.usr.js). Both are now reached
through a single "Administration" sidemenu dropdown (Systemüberwachung /
Benutzerverwaltung) gated by one shared fds_sys > 4 check, per ADR 0015
(supersedes 0014's "fold into one module" choice).

Also fixes the password-reset validation to check only a plain 6-char
minimum instead of silently also requiring ValidatePassword's default
2-uppercase/2-lowercase/2-digit composition, which contradicted the
"mind. 6 Zeichen" message shown to the admin.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Fuchs/wwwroot/web/fis.usr.min.css

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+.usrfrm>table{border-collapse:collapse;margin:2rem}.usrfrm>table td,.usrfrm>table th{padding:.2rem .35rem;border:1px solid #ccc}.usrfrm>table tr{cursor:pointer}.usrfrm>table tr:nth-child(2n+1) td{background-color:#eee}.usrfrm>table tr:hover td{background-color:#f3f6fb}.usrfrm>table tr.selected td{background-color:#add8e6}.usrfrm .usr_err{padding:1rem 2rem;color:#b00020;font-weight:700}.usr_groups_dlg{padding:.25rem 0}.usr_groups_dlg .usr_group_row{display:flex;align-items:center;gap:.5rem;padding:.35rem 0;border-bottom:1px solid #eee}.usr_groups_dlg .usr_group_row:last-child{border-bottom:none}.usr_groups_dlg .usr_group_row input[type=checkbox]{margin:0}.usr_groups_dlg .usr_group_row label{margin:0;font-weight:400}