refactor(client): centralize error feedback with toast events

- emit API/network failures through a shared toast event bridge

- remove per-page inline error notifications to avoid duplicate error UI

- dedupe repeated toast messages to keep feedback readable

client/src/lib/toast-events.ts

@@ -0,0 +1,29 @@
+export const API_ERROR_TOAST_EVENT = 'liqa:api-error-toast';
+
+export interface ApiErrorToastDetail {
+  message: string;
+}
+
+export function emitApiErrorToast(message: string): void {
+  if (typeof window === 'undefined') return;
+  window.dispatchEvent(
+    new CustomEvent<ApiErrorToastDetail>(API_ERROR_TOAST_EVENT, {
+      detail: { message },
+    }),
+  );
+}
+
+export function subscribeApiErrorToasts(
+  handler: (message: string) => void,
+): () => void {
+  if (typeof window === 'undefined') return () => {};
+  const listener = (event: Event) => {
+    const custom = event as CustomEvent<ApiErrorToastDetail>;
+    const message = custom.detail?.message;
+    if (message) handler(message);
+  };
+  window.addEventListener(API_ERROR_TOAST_EVENT, listener as EventListener);
+  return () => {
+    window.removeEventListener(API_ERROR_TOAST_EVENT, listener as EventListener);
+  };
+}