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>

README.md

@@ -575,13 +575,17 @@ office
 
 | Key | Action |
 |---|---|
-| `Tab` | Switch pane (menu → list → preview) |
-| `↑↓` / `j` / `k` | Navigate |
-| `Space` / `Enter` | Toggle selection on list row |
-| `Enter` | Load preview for focused row |
+| `Tab` | Next pane (menu → list → preview) |
+| `Shift+Tab` | Previous pane |
+| `↑↓` / `j` / `k` | Navigate / scroll within focused pane |
+| `Enter` / `→` | Expand tree branch or open leaf list |
+| `Space` | Toggle multi-select on list row |
+| `Enter` / `a` | Action menu (view, delete, download, …) |
 | `r` | Refresh current list |
 | `q` | Quit |
 
+Navigate the **tree** on the left: expand modules (`▸`/`▾`), drill to a **leaf** (marked `•`) — the center list loads only at the last level. Under **Projects → By project**, live projects appear as subnodes with **Tasks** and **Files** leaves.
+
 Optional env for DOCX preview via Document Server (see [`.env.example`](.env.example)):
 
 ```bash