feat(office): scrollable panes, nav tree drill-down, and item actions

Add viewports to all three columns with cursor sync scrolling. Support
Shift+Tab reverse pane focus. Replace flat menu with hierarchical NavTree
(list loads only on leaf nodes; dynamic project subnodes with Tasks/Files).
Enter or a opens per-item action menu (view, delete, download) wired to
the OnlyOffice API.

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

cmd/office/model/item.go

@@ -50,6 +50,18 @@ const (
 	FocusPreview
 )
 
+// PrevFocusPane cycles preview → list → menu → preview.
+func PrevFocusPane(p FocusPane) FocusPane {
+	switch p {
+	case FocusMenu:
+		return FocusPreview
+	case FocusList:
+		return FocusMenu
+	default:
+		return FocusList
+	}
+}
+
 // NextFocusPane cycles menu → list → preview → menu.
 func NextFocusPane(p FocusPane) FocusPane {
 	switch p {