feat(app): checkout diff pane styling and e2e test support

- Style diff file cards edge-to-edge with single divider between files
- Remove monospace font from filenames for better readability
- Add checkout-ship e2e test for app
- Export Page type from e2e fixtures
- Add checkout query hooks for diff, status, and PR status

packages/app/src/utils/extract-agent-model.ts

@@ -3,7 +3,12 @@ import type { Agent } from "@/stores/session-store";
 export function extractAgentModel(agent?: Agent | null): string | null {
   if (!agent) return null;
   const runtimeModel = agent.runtimeInfo?.model;
-  return typeof runtimeModel === "string" && runtimeModel.trim().length > 0
-    ? runtimeModel.trim()
-    : null;
+  const fallbackModel = agent.model;
+  if (typeof runtimeModel === "string" && runtimeModel.trim().length > 0) {
+    return runtimeModel.trim();
+  }
+  if (typeof fallbackModel === "string" && fallbackModel.trim().length > 0) {
+    return fallbackModel.trim();
+  }
+  return null;
 }