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/ui/keys.go

@@ -11,7 +11,9 @@ const (
 	ActionMoveDown
 	ActionToggleSelect
 	ActionOpenPreview
+	ActionOpenActions
 	ActionNextPane
+	ActionPrevPane
 	ActionRefresh
 	ActionQuit
 	ActionOpenVex
@@ -24,6 +26,8 @@ func KeyAction(key string, pane model.FocusPane) Action {
 		return ActionQuit
 	case "tab":
 		return ActionNextPane
+	case "shift+tab", "backtab":
+		return ActionPrevPane
 	case "r":
 		return ActionRefresh
 	case "up", "k":
@@ -34,6 +38,10 @@ func KeyAction(key string, pane model.FocusPane) Action {
 		if pane == model.FocusList {
 			return ActionToggleSelect
 		}
+	case "a":
+		if pane == model.FocusList {
+			return ActionOpenActions
+		}
 	case "enter":
 		if pane == model.FocusList {
 			return ActionOpenPreview