feat(web): sprint1 ui core with global toasts and improved chat layout

web/src/store/uiStore.ts

@@ -0,0 +1,14 @@
+import { create } from "zustand";
+
+interface UiState {
+  toastMessage: string | null;
+  showToast: (message: string) => void;
+  clearToast: () => void;
+}
+
+export const useUiStore = create<UiState>((set) => ({
+  toastMessage: null,
+  showToast: (message) => set({ toastMessage: message }),
+  clearToast: () => set({ toastMessage: null })
+}));
+