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)

internal/store/foldermap_test.go

@@ -0,0 +1,34 @@
+package store
+
+import (
+	"context"
+	"testing"
+)
+
+func TestSetTaskFolderMappingRoundTrip(t *testing.T) {
+	s := testStore(t)
+	ctx := context.Background()
+	e1, _ := s.CreateEndpoint(ctx, Endpoint{RoleLabel: "s", Host: "a", Port: 993, TLSMode: "ssl"})
+	e2, _ := s.CreateEndpoint(ctx, Endpoint{RoleLabel: "d", Host: "b", Port: 993, TLSMode: "ssl"})
+	taskID, _ := s.CreateTask(ctx, Task{Name: "t", SrcEndpointID: e1, DstEndpointID: e2})
+
+	m := map[string]string{"Спам": "Spam", "Отправленные": "Sent"}
+	if err := s.SetTaskFolderMapping(ctx, taskID, m); err != nil {
+		t.Fatalf("set mapping: %v", err)
+	}
+	got, err := s.GetTask(ctx, taskID)
+	if err != nil {
+		t.Fatalf("get: %v", err)
+	}
+	if got.FolderMapping["Спам"] != "Spam" || got.FolderMapping["Отправленные"] != "Sent" {
+		t.Fatalf("mapping round-trip failed: %+v", got.FolderMapping)
+	}
+	// nil clears to empty object, not null
+	if err := s.SetTaskFolderMapping(ctx, taskID, nil); err != nil {
+		t.Fatalf("clear: %v", err)
+	}
+	got, _ = s.GetTask(ctx, taskID)
+	if got.FolderMapping == nil {
+		t.Fatal("mapping should be non-nil empty map after clear")
+	}
+}

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