fix: app login redirect broken after forward_auth challenge

Two bugs:
1. verify() built the login URL with `//login` (double slash) — now `/login`
2. safe_path() rejected absolute https:// next-URLs, so after login the
   user was silently dropped at `/` instead of their original app URL.

Replaced safe_path with safe_redirect(next, domain) which allows relative
paths OR absolute URLs whose host is the configured domain (or a subdomain).
safe_path is kept as a thin wrapper (domain="") for the admin-UI middleware
where next is always a relative path.

https://claude.ai/code/session_01FKCW3FDjNFj6jve4niMFXH

server/src/auth.rs

@@ -60,13 +60,26 @@ pub async fn current_user_id(state: &crate::AppState, headers: &HeaderMap) -> Op
     state.sessions.lock().await.get(&token).cloned()
 }
 
-/// Reject open-redirect: only allow relative paths.
-fn safe_path(next: &str) -> String {
+/// Reject open-redirect: allow relative paths or absolute URLs on the same domain.
+fn safe_redirect(next: &str, domain: &str) -> String {
     if next.starts_with('/') && !next.starts_with("//") {
-        next.to_string()
-    } else {
-        "/".to_string()
+        return next.to_string();
     }
+    // Allow https:// URLs whose host is the domain or a subdomain of it.
+    for prefix in &["https://", "http://"] {
+        if let Some(rest) = next.strip_prefix(prefix) {
+            let host = rest.split('/').next().unwrap_or("");
+            if host == domain || host.ends_with(&format!(".{}", domain)) {
+                return next.to_string();
+            }
+        }
+    }
+    "/".to_string()
+}
+
+/// Reject open-redirect: only allow relative paths (used for admin-UI redirects).
+fn safe_path(next: &str) -> String {
+    safe_redirect(next, "")
 }
 
 // ── Control-plane middleware (admin users only) ───────────────────────────────
@@ -138,7 +151,7 @@ pub async fn verify(
                 format!("{}://{}{}", proto, host, uri)
             };
             let login_url = format!(
-                "https://{}//login?next={}",
+                "https://{}/login?next={}",
                 state.domain_suffix,
                 urlencoding_simple(&next_url)
             );
@@ -232,10 +245,10 @@ pub async fn login_page(
 ) -> Response {
     // Already logged in → redirect.
     if current_user_id(&state, &headers).await.is_some() {
-        let next = params.next.as_deref().map(safe_path).unwrap_or_else(|| "/".into());
+        let next = params.next.as_deref().map(|n| safe_redirect(n, &state.domain_suffix)).unwrap_or_else(|| "/".into());
         return Redirect::to(&next).into_response();
     }
-    let next = params.next.map(|s| safe_path(&s)).unwrap_or_else(|| "/".into());
+    let next = params.next.map(|s| safe_redirect(&s, &state.domain_suffix)).unwrap_or_else(|| "/".into());
     Html(login_html(&next, None)).into_response()
 }
 
@@ -243,7 +256,7 @@ pub async fn handle_login(
     State(state): State<crate::AppState>,
     Form(form): Form<LoginForm>,
 ) -> Response {
-    let next = form.next.as_deref().map(safe_path).unwrap_or_else(|| "/".into());
+    let next = form.next.as_deref().map(|n| safe_redirect(n, &state.domain_suffix)).unwrap_or_else(|| "/".into());
 
     // --- env-var bootstrap path (used before any DB user exists) ---
     let using_bootstrap = matches!(