Fix simulate route crash and surface actual error messages

When a simulation action threw (e.g. readiness check failure), React Router
tried to render the simulate route via GET to display the error — but the
route had no loader, so the real error was completely swallowed by React
Router's own 400 "no loader" message.

Four issues fixed:
- Move simulate into the intent-dispatch pattern on the season page (matching
  delete/rescore/sync-standings/finalize-standings), so errors surface via
  actionData instead of being lost
- Eliminate the ?simulationError= query param approach, which left stale error
  messages in browser history and reflected user-controlled text in the UI
- Match the existing bg-destructive/15 error styling used elsewhere on the page
- Strip the now-unused action from simulate.tsx; keep a loader-only redirect
  stub so any bookmarked URLs degrade gracefully to the season page

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/routes/admin.sports-seasons.$id.simulate.tsx

@@ -1,24 +1,13 @@
 /**
- * Admin: Run EV Simulation for a Sports Season
+ * Admin: Simulate action endpoint (stub)
  *
- * Action-only route. POSTing here:
- *   1. Delegates validation + execution to the shared simulator runner
- *   6. Redirects back to the sports season admin page
+ * Simulation is now handled by intent="simulate" on the season admin page.
+ * This stub redirects any direct GET navigation to that page.
  */
 
 import { redirect } from "react-router";
 import type { Route } from "./+types/admin.sports-seasons.$id.simulate";
-import { runSportsSeasonSimulation } from "~/services/simulations/runner";
 
-export async function action({ params }: Route.ActionArgs) {
-  const sportsSeasonId = params.id;
-  try {
-    await runSportsSeasonSimulation(sportsSeasonId);
-  } catch (error) {
-    throw new Response(error instanceof Error ? error.message : "Simulation failed", {
-      status: error instanceof Error && error.message.includes("already running") ? 409 : 400,
-    });
-  }
-
-  return redirect(`/admin/sports-seasons/${sportsSeasonId}/expected-values`);
+export async function loader({ params }: Route.LoaderArgs) {
+  return redirect(`/admin/sports-seasons/${params.id}`);
 }

app/routes/admin.sports-seasons.$id.tsx

@@ -11,6 +11,7 @@ import { database } from "~/database/context";
 import { participantEvSnapshots, seasonSports } from "~/database/schema";
 import { eq, desc } from "drizzle-orm";
 import { getSimulatorInfo, type SimulatorType } from "~/services/simulations/registry";
+import { runSportsSeasonSimulation } from "~/services/simulations/runner";
 import { syncStandings } from "~/services/standings-sync/index";
 import {
   getPendingStandingsMappings,
@@ -280,6 +281,17 @@ export async function action(args: Route.ActionArgs) {
     }
   }
 
+  if (intent === "simulate") {
+    try {
+      await runSportsSeasonSimulation(params.id);
+      return redirect(`/admin/sports-seasons/${params.id}/expected-values`);
+    } catch (error) {
+      return {
+        simulateError: error instanceof Error ? error.message : "Simulation failed",
+      };
+    }
+  }
+
   // Update
   const name = formData.get("name");
   const year = formData.get("year");
@@ -669,7 +681,8 @@ export default function EditSportsSeason({ loaderData, actionData }: Route.Compo
                     </Button>
                   )}
                   {simulatorInfo && (
-                    <Form method="post" action={`/admin/sports-seasons/${sportsSeason.id}/simulate`}>
+                    <Form method="post">
+                      <input type="hidden" name="intent" value="simulate" />
                       <Button
                         type="submit"
                         size="sm"
@@ -702,6 +715,11 @@ export default function EditSportsSeason({ loaderData, actionData }: Route.Compo
                     </>
                   : "Import futures odds to set probability distributions."}
               </p>
+              {"simulateError" in (actionData ?? {}) && actionData?.simulateError && (
+                <div className="mt-3 bg-destructive/15 text-destructive px-4 py-3 rounded-md text-sm">
+                  {actionData.simulateError}
+                </div>
+              )}
             </CardContent>
           </Card>