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/pages/TaskDetail.tsx

@@ -2,6 +2,7 @@ import { useEffect, useRef, useState, type ChangeEvent, type FormEvent } from 'r
 import { createAccount, deleteAccount, getTask, importCSV, runTask, testAccounts, type TaskDetail as TaskDetailData } from '../api'
 import { connectTaskWS, type TaskEvent } from '../ws'
 import { StatusBadge } from '../components/StatusBadge'
+import { useConfirm } from '../components/ConfirmProvider'
 
 const emptyAccount = { src_login: '', src_pass: '', dst_login: '', dst_pass: '' }
 
@@ -40,6 +41,7 @@ export function TaskDetail({ id }: { id: number }) {
   const [log, setLog] = useState<{ type: string; text: string }[]>([])
   const [form, setForm] = useState(emptyAccount)
   const [busy, setBusy] = useState<'test' | 'run' | 'add' | 'import' | 'delete' | null>(null)
+  const confirm = useConfirm()
   const [error, setError] = useState<string | null>(null)
   const fileInputRef = useRef<HTMLInputElement>(null)
 
@@ -111,7 +113,13 @@ export function TaskDetail({ id }: { id: number }) {
   }
 
   async function onDeleteAccount(accId: number, login: string) {
-    if (!confirm(`Remove account "${login}" from this task?`)) return
+    const ok = await confirm({
+      title: 'Remove account',
+      message: `Remove account "${login}" from this task?`,
+      confirmLabel: 'Remove',
+      danger: true,
+    })
+    if (!ok) return
     setBusy('delete')
     setError(null)
     try {