fix: persist classroom to server for cross-device access

app/classroom/[id]/page.tsx

@@ -27,8 +27,27 @@ export default function ClassroomDetailPage() {
   const generationStartedRef = useRef(false);
 
   const { generateRemaining, retrySingleOutline, stop } = useSceneGenerator({
-    onComplete: () => {
+    onComplete: async () => {
       log.info('[Classroom] All scenes generated');
+      // Update server-side storage with all generated scenes
+      const state = useStageStore.getState();
+      if (state.stage && state.scenes.length > 0) {
+        try {
+          const resp = await fetch('/api/classroom', {
+            method: 'POST',
+            headers: { 'Content-Type': 'application/json' },
+            body: JSON.stringify({
+              stage: state.stage,
+              scenes: state.scenes,
+            }),
+          });
+          if (resp.ok) {
+            log.info('[Classroom] Updated server-side storage with all scenes');
+          }
+        } catch (err) {
+          log.warn('[Classroom] Failed to update server-side storage:', err);
+        }
+      }
     },
   });
 

app/generation-preview/page.tsx

@@ -721,6 +721,29 @@ function GenerationPreviewContent() {
         }),
       );
 
+      // Persist classroom to server-side storage for cross-device access
+      const state = store;
+      try {
+        const persistResp = await fetch('/api/classroom', {
+          method: 'POST',
+          headers: { 'Content-Type': 'application/json' },
+          body: JSON.stringify({
+            stage: state.stage,
+            scenes: state.scenes,
+          }),
+        });
+        if (persistResp.ok) {
+          const persistData = await persistResp.json();
+          log.info('[Generation] Classroom persisted to server:', persistData.url);
+        } else {
+          log.warn(
+            '[Generation] Failed to persist classroom to server, continuing with local-only storage',
+          );
+        }
+      } catch (persistErr) {
+        log.warn('[Generation] Error persisting classroom to server:', persistErr);
+      }
+
       sessionStorage.removeItem('generationSession');
       await store.saveToStorage();
       router.push(`/classroom/${stage.id}`);