feat(ux): toast system, accessible modal wrapper, safer dialogs

- Global toast stack (success/error/info) replacing hidden notice/syncError
- Shared Modal: focus trap, Escape, focus restore, scroll lock; all dialogs migrated
- ConfirmDialog tone prop; limit-override now warning-styled
- Single-entry delete now requires confirmation, per-row busy spinner
- Onboarding no longer closes on failed save; inline error + skip option

src/main.tsx

@@ -1,10 +1,13 @@
 import React from "react";
 import ReactDOM from "react-dom/client";
 import App from "./App";
+import { ToastProvider } from "./components/Toasts";
 import "./index.css";
 
 ReactDOM.createRoot(document.getElementById("root")!).render(
   <React.StrictMode>
-    <App />
+    <ToastProvider>
+      <App />
+    </ToastProvider>
   </React.StrictMode>,
 );