add missing thread state store

src/useThreadState.ts

···
+import { create } from "zustand";
+import { combine } from "zustand/middleware";
+
+export const useThreadState = create(
+  combine(
+    {
+      // Set of collapsed thread URIs
+      collapsedThreads: new Set<string>(),
+    },
+    (set) => ({
+      toggleCollapsed: (uri: string) => {
+        set((state) => {
+          const newCollapsed = new Set(state.collapsedThreads);
+          if (newCollapsed.has(uri)) {
+            newCollapsed.delete(uri);
+          } else {
+            newCollapsed.add(uri);
+          }
+          return { collapsedThreads: newCollapsed };
+        });
+      },
+      isCollapsed: (uri: string) => {
+        // This is a selector helper, but we'll use the state directly
+        return false;
+      },
+    }),
+  ),
+);