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/ui/Toast/ToastProvider.tsx

@@ -1,6 +1,7 @@
-import { createContext, useCallback, useContext, useMemo, useState } from 'react';
+import { createContext, useCallback, useContext, useEffect, useMemo, useRef, useState } from 'react';
 import { AlertCircle, CheckCircle2, Info, X } from 'lucide-react';
 import styles from './ToastProvider.module.css';
+import { subscribeApiErrorToasts } from '../../lib/toast-events';
 
 type ToastVariant = 'success' | 'error' | 'info';
 
@@ -34,6 +35,7 @@ export interface ToastProviderProps {
 export function ToastProvider({ children, durationMs = 3500 }: ToastProviderProps) {
   const EXIT_ANIMATION_MS = 100;
   const [items, setItems] = useState<ToastItem[]>([]);
+  const lastShownAtRef = useRef<Map<string, number>>(new Map());
 
   const remove = useCallback((id: number) => {
     setItems((prev) => prev.filter((item) => item.id !== id));
@@ -49,6 +51,12 @@ export function ToastProvider({ children, durationMs = 3500 }: ToastProviderProp
   }, [remove]);
 
   const show = useCallback((message: string, variant: ToastVariant = 'info') => {
+    const now = Date.now();
+    const dedupeKey = `${variant}:${message}`;
+    const previousTs = lastShownAtRef.current.get(dedupeKey) ?? 0;
+    if (now - previousTs < 600) return;
+    lastShownAtRef.current.set(dedupeKey, now);
+
     const id = Date.now() + Math.floor(Math.random() * 1000);
     setItems((prev) => [...prev, { id, message, variant, closing: false }]);
     window.setTimeout(() => dismiss(id), durationMs);
@@ -61,6 +69,10 @@ export function ToastProvider({ children, durationMs = 3500 }: ToastProviderProp
     info: (message: string) => show(message, 'info'),
   }), [show]);
 
+  useEffect(() => {
+    return subscribeApiErrorToasts((message) => api.error(message));
+  }, [api]);
+
   return (
     <ToastContext.Provider value={api}>
       {children}