Hydrate session PDF byte stores on engine mount.

Create PdfByteStore with sessionId (IndexedDB) and re-render after hydrate
so the viewer gets live blob URLs after reload.

src/work/EngineContext.tsx

@@ -153,6 +153,23 @@ export function EngineProvider({
     return attachPersister(engine, { key: snapshotKey });
   }, [engine, injected, snapshotKey, activeDocKey]);
 
+  // Rehydrate PDF bytes from IndexedDB (session-scoped). Snapshot restore
+  // only brings back metadata; without this the viewer points at a dead
+  // blob / fake fixture path and PDF.js reports "Invalid PDF structure".
+  useEffect(() => {
+    if (injected) return;
+    let cancelled = false;
+    void (async () => {
+      await byteStore.hydrate();
+      if (!cancelled) {
+        setEngineRevision((n) => n + 1);
+      }
+    })();
+    return () => {
+      cancelled = true;
+    };
+  }, [byteStore, injected, sessionId]);
+
   // Persist the active-document pointer alongside the engine snapshot so a
   // reload lands the user back where they were.
   useEffect(() => {

src/work/SessionContext.tsx

@@ -80,7 +80,8 @@ export function SessionProvider({
     (sessionId: SessionId) => {
       let store = byteStores.get(sessionId);
       if (!store) {
-        store = createPdfByteStore();
+        // sessionId enables IndexedDB mirror so PDF bytes survive reloads.
+        store = createPdfByteStore({ sessionId });
         byteStores.set(sessionId, store);
       }
       return store;