feat(errors): per-account error modal with persisted error list

Accounts finishing done_with_errors showed only a count and a single
last_error. This adds a modal listing every concrete error of the
account's most recent run.

- migration 0005: account_errors table (kind folder|message|account,
  folder, message_ref, error, created_at; ON DELETE CASCADE; indexed)
- store: AddAccountError / ClearAccountErrors / ListAccountErrors
- copy: OnError callback captures per-message error text (previously
  only counted), with a "UID N: subject" reference
- orchestrator: clear errors at run start; persist folder/message/
  account errors; cap 500 rows/account/run with a suppressed-note row
- api: GET /api/tasks/{id}/accounts/{accountId}/errors
- web: AccountErrorsModal, clickable ERRORS count, api + styles

Verified: migration applies on Postgres 18; store add/list/clear and
cascade tests pass against real pg; backend build/vet/test green; web
tsc+vite build and oxlint clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U9Eq4JtWjyNTv5qat3B3mM

web/src/components/AccountErrorsModal.tsx

@@ -0,0 +1,65 @@
+import { useEffect, useState } from 'react'
+import { Modal } from './Modal'
+import { listAccountErrors, type AccountError } from '../api'
+
+const fmt = (iso: string) => (iso ? new Date(iso).toLocaleString() : '—')
+
+export function AccountErrorsModal({
+  taskId,
+  account,
+  onClose,
+}: {
+  taskId: number
+  account: { id: number; src_login: string } | null
+  onClose: () => void
+}) {
+  const [errors, setErrors] = useState<AccountError[] | null>(null)
+  const [failed, setFailed] = useState(false)
+  const open = account !== null
+
+  useEffect(() => {
+    if (!account) return
+    setErrors(null)
+    setFailed(false)
+    listAccountErrors(taskId, account.id)
+      .then((e) => setErrors(e ?? []))
+      .catch(() => setFailed(true))
+  }, [taskId, account])
+
+  return (
+    <Modal open={open} title={account ? `Errors — ${account.src_login}` : 'Errors'} onClose={onClose} size="lg">
+      <div className="tbl-wrap">
+        <table className="tbl">
+          <thead>
+            <tr>
+              <th>Time</th>
+              <th>Kind</th>
+              <th>Folder</th>
+              <th>Message</th>
+              <th>Error</th>
+            </tr>
+          </thead>
+          <tbody>
+            {failed ? (
+              <tr className="empty-row"><td colSpan={5}>failed to load errors</td></tr>
+            ) : errors === null ? (
+              <tr className="empty-row"><td colSpan={5}>loading…</td></tr>
+            ) : errors.length === 0 ? (
+              <tr className="empty-row"><td colSpan={5}>no errors recorded</td></tr>
+            ) : (
+              errors.map((e) => (
+                <tr key={e.id}>
+                  <td>{fmt(e.created_at)}</td>
+                  <td>{e.kind}</td>
+                  <td>{e.folder || '—'}</td>
+                  <td>{e.message_ref || '—'}</td>
+                  <td className="err-cell">{e.error}</td>
+                </tr>
+              ))
+            )}
+          </tbody>
+        </table>
+      </div>
+    </Modal>
+  )
+}