Fix simulate route crash and surface actual simulation errors (#64)

## Problem

Running a simulation on MLB (and any sport where the readiness check fails) showed a confusing React Router 400 error instead of the real failure reason. The cause: the simulate route had only an \`action\` export. When the action **threw** instead of redirecting, React Router tried to render the route via GET to display the error — hit the missing loader — and replaced the real error with its own internal 400.

A secondary issue: the interim fix used \`?simulationError=\` query params, which had three problems identified in code review:
- Stale errors re-appeared when pressing browser Back after fixing the issue
- The param reflected arbitrary text into the UI (social engineering surface)
- The styling didn't match the established \`bg-destructive/15\` error pattern

## Changes

**\`admin.sports-seasons.\$id.simulate.tsx\`** — strip the action; keep a loader-only redirect stub so bookmarked URLs degrade gracefully to the season page.

**\`admin.sports-seasons.\$id.tsx\`** — add \`intent="simulate"\` to the action dispatcher (matching \`delete\`, \`rescore\`, \`sync-standings\`, \`finalize-standings\`). On success: redirect to expected-values (unchanged). On failure: return \`{ simulateError: message }\` displayed inline with the correct \`bg-destructive/15\` styling.

## Test plan

- [ ] Click Run Simulation on a season with a missing readiness input → error message appears inline below the description text, styled consistently with other errors on the page
- [ ] Click Run Simulation on a fully-configured season → redirects to expected-values as before
- [ ] Navigate directly to \`/admin/sports-seasons/<id>/simulate\` in browser → redirects to season page (no 400)
- [ ] Run simulation, see error, fix the issue, run again successfully → no stale error message persists

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Chris Parsons <chrisparsons1127@gmail.com>
Reviewed-on: #64

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>