feat(web): reusable Modal + ConfirmProvider, replace native confirm()

Modal component: portal, ESC to close, Tab focus-trap, focus-on-open
(prefers [data-modal-autofocus]), focus restore, overlay click, scroll lock.
ConfirmProvider exposes useConfirm(): async confirm({...}) as a drop-in for
window.confirm; Enter confirms, ESC cancels. Task/account deletes now use it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMHQTtnQtQqL8muAXHr9kd

web/src/main.tsx

@@ -2,9 +2,12 @@ import { StrictMode } from 'react'
 import { createRoot } from 'react-dom/client'
 import './index.css'
 import App from './App.tsx'
+import { ConfirmProvider } from './components/ConfirmProvider'
 
 createRoot(document.getElementById('root')!).render(
   <StrictMode>
-    <App />
+    <ConfirmProvider>
+      <App />
+    </ConfirmProvider>
   </StrictMode>,
 )