POC(nextcloud): add chroot on nextcloud

Bring chroot to nextcloud capability: login on Nextcloud via username="{user}~{folder-uuid}
    Doing a such login will chroot the folder folder-uuid

    If user has several folder as root (parent=None), the login flow will request which
    folder user want to chroot

src/interfaces/nextcloud/avatar_handler.rs

@@ -101,6 +101,11 @@ pub async fn handle_avatar(
 ) -> Response {
     let size = size.clamp(16, 1024);
 
+    let username = match username.split_once("~") {
+        None => username,
+        Some((u, _)) => u.to_string(),
+    };
+
     // ── Stored profile image — preferred when present ───────────
     if let Some(auth_svc) = state.auth_service.as_ref()
         && let Ok(user) = auth_svc