fix(ui): the backend picker showed two options meaning the same thing

`default` is the generic `rootfs.ext4` and `claude` is the named one, and
`microvm_credential_for` gives them the identical contract — so the list came
back with both under the same label, and whichever a user picked they got the
same thing. Collapsed to the named one where it exists; the generic keeps a
label of its own for a fleet that only has that.

Co-Authored-By: Claude Opus 5 <[email protected]>

crates/cm-api/src/routes/nodes.rs

@@ -479,12 +479,19 @@ pub async fn backends(
     Authed(user): Authed,
 ) -> Result<Json<Value>, ApiError> {
     let ws = user.workspace_id.as_uuid().to_owned();
-    let list = crate::mission_roster::available_backends(&state.pool, ws)
+    let mut list = crate::mission_roster::available_backends(&state.pool, ws)
         .await
         .map_err(|e| {
             eprintln!("fleet backends: {e}");
             ApiError::Internal
         })?;
+    // `default` is the generic `rootfs.ext4` and `claude` is the named one, and
+    // `microvm_credential_for` gives them the SAME contract — so a picker
+    // offering both shows two options with one meaning, and whichever the user
+    // picks they get the same thing. Collapse to the named one where it exists.
+    if list.iter().any(|b| b == "claude") {
+        list.retain(|b| b != "default");
+    }
     Ok(Json(json!({
         "backends": list.iter().map(|b| json!({
             "id": b,
@@ -498,7 +505,8 @@ pub async fn backends(
 /// to know which company each one bills.
 fn backend_label(id: &str) -> String {
     match id {
-        "claude" | "default" => "Claude (Anthropic subscription)".into(),
+        "claude" => "Claude (Anthropic subscription)".into(),
+        "default" => "Claude (generic image)".into(),
         "canary-claude" => "Claude — candidate CLI (canary)".into(),
         "glm" => "GLM 4.7 (z.ai)".into(),
         "kimi" => "Kimi (Moonshot)".into(),