fix(webapp): address Devin review findings on schedules sheet

- Schedule detail action: honor `_format=json` in the project-not-found
  guard so fetcher callers get a structured error envelope.
- ScheduleSheet: treat schedule data as loading when its friendlyId
  doesn't match the currently open id (fixes stale-data flash when
  switching schedules).
- ScheduleSheet: render an explicit "schedule no longer exists" panel
  when the loader returns `null`, instead of an infinite spinner.

Author: samejr

apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.schedules.$scheduleParam/route.tsx

@@ -85,6 +85,10 @@ export const action = async ({ request, params }: ActionFunctionArgs) => {
   });
 
   if (!project) {
+    const message = `No project found with slug ${projectParam}`;
+    if (wantsJson) {
+      return json({ ok: false as const, message }, { status: 404 });
+    }
     return redirectWithErrorMessage(
       v3SchedulePath(
         { slug: organizationSlug },
@@ -93,7 +97,7 @@ export const action = async ({ request, params }: ActionFunctionArgs) => {
         { friendlyId: scheduleParam }
       ),
       request,
-      `No project found with slug ${projectParam}`
+      message
     );
   }
 

apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.tasks.scheduled.$taskParam/route.tsx

@@ -638,8 +638,18 @@ function ScheduleSheet({
   }, [deleteFetcher.state, deleteFetcher.data, toast, revalidator, onClose]);
 
   const schedule = detailFetcher.data?.schedule;
+  // Treat stale data (previous schedule still in fetcher cache after the
+  // user clicked a different row) as loading — otherwise we briefly flash
+  // the previous schedule's content while the new fetch is in flight.
+  const isStaleSchedule = !!schedule && !!openScheduleId && schedule.friendlyId !== openScheduleId;
   const isDetailLoading =
-    detailFetcher.state === "loading" || (!!openScheduleId && !schedule);
+    detailFetcher.state === "loading" ||
+    isStaleSchedule ||
+    (!!openScheduleId && schedule === undefined);
+  // Distinct from loading: the loader has resolved and the schedule is
+  // genuinely gone (returned `null`, e.g. deleted externally).
+  const isScheduleMissing =
+    !!openScheduleId && !isDetailLoading && detailFetcher.data?.schedule === null;
   const editData = editFetcher.data;
   const isEditLoading =
     mode === "edit" && (editFetcher.state === "loading" || !editData);
@@ -665,16 +675,20 @@ function ScheduleSheet({
               submitFetcher={updateFetcher}
             />
           )
-        ) : isDetailLoading || !schedule ? (
+        ) : isDetailLoading ? (
           <TableLoading />
-        ) : (
+        ) : isScheduleMissing ? (
+          <ScheduleMissingPanel onClose={onClose} />
+        ) : schedule ? (
           <ScheduleInspector
             schedule={schedule}
             actionPath={detailPath}
             onEdit={() => setMode("edit")}
             activeToggleFetcher={activeToggleFetcher}
             deleteFetcher={deleteFetcher}
           />
+        ) : (
+          <TableLoading />
         )}
       </SheetContent>
     </Sheet>
@@ -1098,3 +1112,16 @@ function TableLoading() {
     </div>
   );
 }
+
+function ScheduleMissingPanel({ onClose }: { onClose: () => void }) {
+  return (
+    <div className="flex h-full flex-col items-center justify-center gap-3 bg-background-bright p-6 text-center">
+      <Paragraph variant="small" className="text-text-bright">
+        This schedule no longer exists.
+      </Paragraph>
+      <Button variant="secondary/small" onClick={onClose}>
+        Close
+      </Button>
+    </div>
+  );
+}