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/httpapi/accounts.go

@@ -7,6 +7,7 @@ import (
 	"strings"
 
 	"github.com/vasyansk/imap-copier/internal/crypto"
+	"github.com/vasyansk/imap-copier/internal/imapx"
 	"github.com/vasyansk/imap-copier/internal/store"
 )
 
@@ -30,6 +31,76 @@ func accountDTO(a store.Account) AccountView {
 	}
 }
 
+// handleProbeFolders tests both logins with the given credentials and returns
+// the folder list on each side, so the UI can offer a source->destination
+// folder mapping before the account is created. Credentials are not stored.
+func (s *Server) handleProbeFolders(w http.ResponseWriter, r *http.Request) {
+	taskID, err := pathID(r, "id")
+	if err != nil {
+		http.Error(w, "bad id", http.StatusBadRequest)
+		return
+	}
+	task, err := s.store.GetTask(r.Context(), taskID)
+	if err != nil {
+		http.Error(w, "not found", http.StatusNotFound)
+		return
+	}
+	srcEP, err := s.store.GetEndpoint(r.Context(), task.SrcEndpointID)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	dstEP, err := s.store.GetEndpoint(r.Context(), task.DstEndpointID)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	var body struct {
+		SrcLogin string `json:"src_login"`
+		SrcPass  string `json:"src_pass"`
+		DstLogin string `json:"dst_login"`
+		DstPass  string `json:"dst_pass"`
+	}
+	if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
+		http.Error(w, "bad json", http.StatusBadRequest)
+		return
+	}
+	probe := func(ep store.Endpoint, login, pass string) map[string]any {
+		folders, err := imapx.TestLogin(r.Context(),
+			imapx.Endpoint{Host: ep.Host, Port: ep.Port, TLSMode: ep.TLSMode},
+			strings.TrimSpace(login), strings.TrimSpace(pass))
+		if err != nil {
+			return map[string]any{"ok": false, "error": err.Error(), "folders": []string{}}
+		}
+		return map[string]any{"ok": true, "folders": folders}
+	}
+	writeJSON(w, http.StatusOK, map[string]any{
+		"src": probe(srcEP, body.SrcLogin, body.SrcPass),
+		"dst": probe(dstEP, body.DstLogin, body.DstPass),
+	})
+}
+
+// handleSetFolderMapping replaces the task's src->dst folder mapping.
+func (s *Server) handleSetFolderMapping(w http.ResponseWriter, r *http.Request) {
+	taskID, err := pathID(r, "id")
+	if err != nil {
+		http.Error(w, "bad id", http.StatusBadRequest)
+		return
+	}
+	var body struct {
+		Mapping map[string]string `json:"mapping"`
+	}
+	if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
+		http.Error(w, "bad json", http.StatusBadRequest)
+		return
+	}
+	if err := s.store.SetTaskFolderMapping(r.Context(), taskID, body.Mapping); err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	w.WriteHeader(http.StatusNoContent)
+}
+
 func pathID(r *http.Request, name string) (int64, error) {
 	return strconv.ParseInt(r.PathValue(name), 10, 64)
 }

internal/httpapi/router.go

@@ -20,6 +20,8 @@ func (s *Server) Router() http.Handler {
 	api.HandleFunc("GET /api/tasks/{id}", s.handleGetTask)
 	api.HandleFunc("DELETE /api/tasks/{id}", s.handleDeleteTask)
 	api.HandleFunc("POST /api/tasks/{id}/accounts", s.handleCreateAccount)
+	api.HandleFunc("POST /api/tasks/{id}/probe", s.handleProbeFolders)
+	api.HandleFunc("PUT /api/tasks/{id}/folder-mapping", s.handleSetFolderMapping)
 	api.HandleFunc("DELETE /api/tasks/{id}/accounts/{accountId}", s.handleDeleteAccount)
 	api.HandleFunc("POST /api/tasks/{id}/import", s.handleImportCSV)
 	api.HandleFunc("POST /api/tasks/{id}/test", s.handleTestAccounts)