Add PDF modal and selectedPost setter

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/store/useAppStore.ts

@@ -18,6 +18,7 @@ type AppState = {
   endorseUser: (userId: string, specialty: string) => void
   endorsePost: (postId: string) => void
   attachPDFToPost: (postId: string, file: File) => void
+  setSelectedPost: (id: string | null) => void
   toggleCreatePost: () => void
 }
 
@@ -91,6 +92,8 @@ const useAppStore = create<AppState>((set, get) => ({
     }))
   },
 
+  setSelectedPost: (id) => set(() => ({ selectedPostId: id })),
+
   toggleCreatePost: () => set((state) => ({ ui: { isCreatePostOpen: !state.ui.isCreatePostOpen } })),
 }))