feat: folder mapping UI on account add (probe + map modal)

ADD now probes both connections, lists folders on each side, and opens a
mapping modal to route source->destination folders (e.g. Спам -> Spam) so we
append into the existing folder instead of creating a duplicate.

- store: SetTaskFolderMapping (+ round-trip test)
- httpapi: POST /tasks/{id}/probe (test both, return folder lists),
  PUT /tasks/{id}/folder-mapping
- web: FolderMappingModal (reuses Modal, size=lg), submitAccount probes then
  opens the modal; confirm creates the account and saves the task mapping

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

@@ -1,8 +1,9 @@
 import { useEffect, useRef, useState, type ChangeEvent, type FormEvent } from 'react'
-import { cancelAccount, createAccount, deleteAccount, getTask, importCSV, runTask, testAccounts, type TaskDetail as TaskDetailData } from '../api'
+import { cancelAccount, createAccount, deleteAccount, getTask, importCSV, probeFolders, runTask, setFolderMapping, testAccounts, type TaskDetail as TaskDetailData } from '../api'
 import { connectTaskWS, type TaskEvent } from '../ws'
 import { StatusBadge } from '../components/StatusBadge'
 import { useConfirm } from '../components/ConfirmProvider'
+import { FolderMappingModal } from '../components/FolderMappingModal'
 
 const emptyAccount = { src_login: '', src_pass: '', dst_login: '', dst_pass: '' }
 
@@ -72,7 +73,8 @@ export function TaskDetail({ id }: { id: number }) {
   const [notFound, setNotFound] = useState(false)
   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 [busy, setBusy] = useState<'test' | 'run' | 'add' | 'import' | 'delete' | 'probe' | null>(null)
+  const [mapState, setMapState] = useState<{ src: string[]; dst: string[]; creds: typeof emptyAccount } | null>(null)
   const confirm = useConfirm()
   const [error, setError] = useState<string | null>(null)
   const [live, setLive] = useState<Record<number, LiveProgress>>({})
@@ -164,11 +166,35 @@ export function TaskDetail({ id }: { id: number }) {
 
   async function submitAccount(e: FormEvent) {
     e.preventDefault()
+    setBusy('probe')
+    setError(null)
+    try {
+      const creds = { ...form }
+      const res = await probeFolders(id, creds)
+      if (!res.src.ok || !res.dst.ok) {
+        const parts: string[] = []
+        if (!res.src.ok) parts.push(`source: ${res.src.error ?? 'login failed'}`)
+        if (!res.dst.ok) parts.push(`destination: ${res.dst.error ?? 'login failed'}`)
+        setError(`Connection test failed — ${parts.join('; ')}`)
+        return
+      }
+      setMapState({ src: res.src.folders ?? [], dst: res.dst.folders ?? [], creds })
+    } catch (err) {
+      setError(err instanceof Error ? err.message : 'Failed to test connections')
+    } finally {
+      setBusy(null)
+    }
+  }
+
+  async function confirmMapping(mapping: Record<string, string>) {
+    if (!mapState) return
     setBusy('add')
     setError(null)
     try {
-      await createAccount(id, form)
+      await createAccount(id, mapState.creds)
+      await setFolderMapping(id, mapping)
       setForm(emptyAccount)
+      setMapState(null)
       reload()
     } catch (err) {
       setError(err instanceof Error ? err.message : 'Failed to add account')
@@ -382,7 +408,7 @@ export function TaskDetail({ id }: { id: number }) {
             </div>
             <div className="btn-row">
               <button className="btn btn-primary" disabled={busy !== null}>
-                {busy === 'add' ? 'Adding…' : 'Add account'}
+                {busy === 'probe' ? 'Testing…' : busy === 'add' ? 'Adding…' : 'Add account'}
               </button>
             </div>
           </form>
@@ -510,6 +536,15 @@ export function TaskDetail({ id }: { id: number }) {
           </table>
         </div>
       </div>
+
+      <FolderMappingModal
+        open={mapState !== null}
+        srcFolders={mapState?.src ?? []}
+        dstFolders={mapState?.dst ?? []}
+        initialMapping={task.folder_mapping ?? {}}
+        onCancel={() => setMapState(null)}
+        onConfirm={confirmMapping}
+      />
     </>
   )
 }