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

internal/store/tasks.go

@@ -57,6 +57,14 @@ func (s *Store) ListTasks(ctx context.Context) ([]Task, error) {
 	return out, rows.Err()
 }
 
+func (s *Store) SetTaskFolderMapping(ctx context.Context, id int64, mapping map[string]string) error {
+	if mapping == nil {
+		mapping = map[string]string{}
+	}
+	_, err := s.Pool.Exec(ctx, `UPDATE tasks SET folder_mapping=$2 WHERE id=$1`, id, mapping)
+	return err
+}
+
 func (s *Store) SetTaskStatus(ctx context.Context, id int64, status string) error {
 	_, err := s.Pool.Exec(ctx, `UPDATE tasks SET status=$2 WHERE id=$1`, id, status)
 	return err