Say when a usage 401 is an expired login, not an unreachable endpoint

A 401 is the endpoint answering and refusing the stored OAuth token, which
Claude Code refreshes as it runs -- so a machine whose CLI has been idle
hands us a stale one. Reporting it as "usage endpoint unreachable" pointed
at the network instead of at the one thing that fixes it.

server/src/usage.rs

@@ -206,15 +206,8 @@ impl UsageProvider for ClaudeUsage {
             .and_then(|mut response| response.body_mut().read_to_string())
         {
             Ok(text) => text,
-            Err(err) => {
-                // The error string can embed the URL but never the token.
-                return self.snapshot(
-                    UsageState::Failed {
-                        detail: format!("usage endpoint unreachable: {err}"),
-                    },
-                    Vec::new(),
-                );
-            }
+            // The error string can embed the URL but never the token.
+            Err(err) => return self.snapshot(UsageState::Failed { detail: why(&err) }, Vec::new()),
         };
         let body: Value = match serde_json::from_str(&text) {
             Ok(body) => body,
@@ -231,6 +224,25 @@ impl UsageProvider for ClaudeUsage {
     }
 }
 
+/// What a failed call to the usage endpoint should say.
+///
+/// A status is not a network fault and must not be reported as one: the
+/// endpoint answered, and 401 in particular says the stored token has expired
+/// -- Claude Code refreshes it as it runs, so a machine whose CLI has been
+/// idle long enough hands us a stale one. That is fixable, and the message is
+/// the only place anybody finds out how.
+fn why(err: &ureq::Error) -> String {
+    match err {
+        ureq::Error::StatusCode(401) => {
+            format!(
+                "the Claude login on this machine has expired (401); run `claude` there, or re-run `/login`, to refresh {CREDENTIALS}"
+            )
+        }
+        ureq::Error::StatusCode(code) => format!("usage endpoint refused the request: HTTP {code}"),
+        other => format!("usage endpoint unreachable: {other}"),
+    }
+}
+
 /// Which kind of "no credentials" a failed read was.
 ///
 /// The distinction is the point of having both states. `cat` failing because
@@ -631,6 +643,15 @@ mod tests {
     use super::*;
     use crate::config::DriverKind;
 
+    #[test]
+    fn an_expired_login_is_not_reported_as_an_unreachable_endpoint() {
+        let stale = why(&ureq::Error::StatusCode(401));
+        assert!(stale.contains("expired"), "{stale}");
+        assert!(!stale.contains("unreachable"), "{stale}");
+        assert!(why(&ureq::Error::StatusCode(500)).contains("HTTP 500"));
+        assert!(why(&ureq::Error::HostNotFound).contains("unreachable"));
+    }
+
     #[test]
     fn parses_the_limits_array_defensively() {
         // Trimmed from a live 2026-08-24 response.