feat(office): table detail panes and Alt+1/2/3 layout toggles

Add spreadsheet-style list, split detail pane with forms and document preview,
and Alt+1/2/3 to show/hide panes with even width distribution.

Co-authored-by: Cursor <cursoragent@cursor.com>

cmd/office/fetch/preview.go

@@ -0,0 +1,43 @@
+package fetch
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+
+	onlyoffice "github.com/eslider/go-onlyoffice"
+	"github.com/eslider/go-onlyoffice/cmd/office/model"
+	"github.com/eslider/go-onlyoffice/cmd/office/preview"
+)
+
+// PreviewMarkdown loads item data and returns markdown for the preview pane.
+func (l *Loader) PreviewMarkdown(ctx context.Context, item model.Item) (string, error) {
+	if l == nil || l.Client == nil {
+		return "", fmt.Errorf("fetch: client is nil")
+	}
+	if item.Kind == model.KindFile {
+		return l.filePreviewMarkdown(ctx, item)
+	}
+	raw, err := l.Detail(ctx, item)
+	if err != nil {
+		return "", err
+	}
+	return preview.EntityMarkdown(string(item.Kind), raw), nil
+}
+
+func (l *Loader) filePreviewMarkdown(ctx context.Context, item model.Item) (string, error) {
+	if item.ID == "" {
+		return "", fmt.Errorf("file id missing")
+	}
+	name := item.Title
+	if meta, err := l.Client.GetFile(ctx, item.ID); err == nil && meta != nil {
+		if t := onlyoffice.FileEntryTitle(meta); t != "" {
+			name = t
+		}
+	}
+	var buf bytes.Buffer
+	if _, err := l.Client.DownloadFile(ctx, item.ID, &buf); err != nil {
+		return "", err
+	}
+	return preview.FileBytesToMarkdown(name, buf.Bytes())
+}