feat: add session-based storage for chat training comparison handoff

- Implemented a utility to manage `training-compare-handoff` data in `sessionStorage` with strict validation and expiration logic.
- Added methods to set, retrieve, and clear handoff data for improved chat training flow.

studio/frontend/src/features/chat/lib/training-compare-handoff.ts

@@ -0,0 +1,53 @@
+const TRAINING_COMPARE_HANDOFF_KEY = "chat:training-compare-handoff:v1";
+const HANDOFF_MAX_AGE_MS = 15 * 60 * 1000;
+
+export type TrainingCompareHandoff = {
+  intent: "compare";
+  baseModel: string | null;
+  requestedAt: number;
+};
+
+export function setTrainingCompareHandoff(baseModel: string | null): void {
+  if (typeof window === "undefined") return;
+
+  const payload: TrainingCompareHandoff = {
+    intent: "compare",
+    baseModel,
+    requestedAt: Date.now(),
+  };
+  window.sessionStorage.setItem(
+    TRAINING_COMPARE_HANDOFF_KEY,
+    JSON.stringify(payload),
+  );
+}
+
+export function getTrainingCompareHandoff(): TrainingCompareHandoff | null {
+  if (typeof window === "undefined") return null;
+
+  const raw = window.sessionStorage.getItem(TRAINING_COMPARE_HANDOFF_KEY);
+  if (!raw) return null;
+
+  try {
+    const parsed = JSON.parse(raw) as Partial<TrainingCompareHandoff>;
+    if (parsed.intent !== "compare") return null;
+    if (typeof parsed.requestedAt !== "number") return null;
+    if (Date.now() - parsed.requestedAt > HANDOFF_MAX_AGE_MS) {
+      clearTrainingCompareHandoff();
+      return null;
+    }
+    return {
+      intent: "compare",
+      baseModel:
+        typeof parsed.baseModel === "string" ? parsed.baseModel : null,
+      requestedAt: parsed.requestedAt,
+    };
+  } catch {
+    clearTrainingCompareHandoff();
+    return null;
+  }
+}
+
+export function clearTrainingCompareHandoff(): void {
+  if (typeof window === "undefined") return;
+  window.sessionStorage.removeItem(TRAINING_COMPARE_HANDOFF_KEY);
+}