feat: add Toast notification system

- ToastProvider context with useToast hook
- Supports success, error, info, and warning variants
- Auto-dismiss with configurable duration (default 4s)
- Slide-in animation with progress bar
- Stacks up to 5 toasts, oldest dismissed first
- Consistent with dark theme UI

client/src/components/ToastProvider.jsx

@@ -0,0 +1,145 @@
+import React, { createContext, useContext, useState, useCallback, useRef, useEffect } from 'react';
+
+const ToastContext = createContext(null);
+
+export function useToast() {
+  const ctx = useContext(ToastContext);
+  if (!ctx) throw new Error('useToast must be used within a ToastProvider');
+  return ctx;
+}
+
+const VARIANTS = {
+  success: { bg: '#053321', border: '#0f5132', color: '#9ef7c1', icon: '✓' },
+  error:   { bg: '#3c1114', border: '#f5c6cb', color: '#ffb3b8', icon: '✗' },
+  info:    { bg: '#0c1f3f', border: '#2563eb', color: '#93c5fd', icon: 'ℹ' },
+  warning: { bg: '#3b2e00', border: '#d4af37', color: '#ffdf8a', icon: '⚠' },
+};
+
+let nextId = 0;
+
+function Toast({ toast, onDismiss }) {
+  const v = VARIANTS[toast.variant] || VARIANTS.info;
+  const [exiting, setExiting] = useState(false);
+  const timerRef = useRef(null);
+
+  useEffect(() => {
+    timerRef.current = setTimeout(() => {
+      setExiting(true);
+      setTimeout(() => onDismiss(toast.id), 280);
+    }, toast.duration || 4000);
+    return () => clearTimeout(timerRef.current);
+  }, [toast.id, toast.duration, onDismiss]);
+
+  const handleDismiss = () => {
+    clearTimeout(timerRef.current);
+    setExiting(true);
+    setTimeout(() => onDismiss(toast.id), 280);
+  };
+
+  return (
+    <div style={{
+      background: v.bg,
+      border: `1px solid ${v.border}`,
+      borderRadius: '8px',
+      padding: '12px 16px',
+      display: 'flex',
+      alignItems: 'flex-start',
+      gap: '10px',
+      color: v.color,
+      fontSize: '13px',
+      fontWeight: 500,
+      minWidth: '320px',
+      maxWidth: '480px',
+      boxShadow: '0 4px 24px rgba(0,0,0,0.5)',
+      animation: exiting ? 'toastOut 0.28s ease-in forwards' : 'toastIn 0.28s ease-out',
+      position: 'relative',
+      overflow: 'hidden',
+    }}>
+      <span style={{ fontSize: '16px', lineHeight: 1, flexShrink: 0, marginTop: '1px' }}>{v.icon}</span>
+      <span style={{ flex: 1, lineHeight: 1.5 }}>{toast.message}</span>
+      <button
+        onClick={handleDismiss}
+        style={{
+          background: 'none', border: 'none', color: v.color, cursor: 'pointer',
+          fontSize: '16px', padding: '0 0 0 8px', opacity: 0.7, lineHeight: 1, flexShrink: 0,
+        }}
+        aria-label="Dismiss"
+      >
+        ×
+      </button>
+      <div style={{
+        position: 'absolute', bottom: 0, left: 0, height: '3px',
+        background: v.color, opacity: 0.4, borderRadius: '0 0 8px 8px',
+        animation: `toastProgress ${toast.duration || 4000}ms linear forwards`,
+      }} />
+    </div>
+  );
+}
+
+export default function ToastProvider({ children }) {
+  const [toasts, setToasts] = useState([]);
+
+  const dismiss = useCallback((id) => {
+    setToasts(prev => prev.filter(t => t.id !== id));
+  }, []);
+
+  const addToast = useCallback((message, variant = 'info', duration = 4000) => {
+    const id = ++nextId;
+    setToasts(prev => {
+      const next = [...prev, { id, message, variant, duration }];
+      return next.length > 5 ? next.slice(-5) : next;
+    });
+    return id;
+  }, []);
+
+  const toast = useCallback({
+    success: (msg, dur) => addToast(msg, 'success', dur),
+    error:   (msg, dur) => addToast(msg, 'error', dur || 6000),
+    info:    (msg, dur) => addToast(msg, 'info', dur),
+    warning: (msg, dur) => addToast(msg, 'warning', dur || 5000),
+  }, [addToast]);
+
+  // Inject keyframes once
+  useEffect(() => {
+    if (document.getElementById('toast-keyframes')) return;
+    const style = document.createElement('style');
+    style.id = 'toast-keyframes';
+    style.textContent = `
+      @keyframes toastIn {
+        from { opacity: 0; transform: translateX(100%); }
+        to   { opacity: 1; transform: translateX(0); }
+      }
+      @keyframes toastOut {
+        from { opacity: 1; transform: translateX(0); }
+        to   { opacity: 0; transform: translateX(100%); }
+      }
+      @keyframes toastProgress {
+        from { width: 100%; }
+        to   { width: 0%; }
+      }
+    `;
+    document.head.appendChild(style);
+  }, []);
+
+  return (
+    <ToastContext.Provider value={toast}>
+      {children}
+      <div style={{
+        position: 'fixed',
+        top: '16px',
+        right: '16px',
+        zIndex: 99999,
+        display: 'flex',
+        flexDirection: 'column',
+        gap: '8px',
+        pointerEvents: 'none',
+      }}>
+        {toasts.map(t => (
+          <div key={t.id} style={{ pointerEvents: 'auto' }}>
+            <Toast toast={t} onDismiss={dismiss} />
+          </div>
+        ))}
+      </div>
+    </ToastContext.Provider>
+  );
+}