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/api.ts

@@ -84,6 +84,25 @@ export const deleteAccount = (taskId: number, accountId: number) =>
 export const cancelAccount = (taskId: number, accountId: number) =>
   api(`/api/tasks/${taskId}/accounts/${accountId}/cancel`, { method: 'POST' })
 
+export interface ProbeSide {
+  ok: boolean
+  folders?: string[]
+  error?: string
+}
+
+export interface ProbeResult {
+  src: ProbeSide
+  dst: ProbeSide
+}
+
+export const probeFolders = (
+  taskId: number,
+  creds: { src_login: string; src_pass: string; dst_login: string; dst_pass: string },
+) => api<ProbeResult>(`/api/tasks/${taskId}/probe`, jsonBody(creds))
+
+export const setFolderMapping = (taskId: number, mapping: Record<string, string>) =>
+  api(`/api/tasks/${taskId}/folder-mapping`, { ...jsonBody({ mapping }), method: 'PUT' })
+
 export const listTasks = () => api<Task[]>('/api/tasks')
 
 export const getTask = (id: number) => api<TaskDetail>(`/api/tasks/${id}`)