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/nav_projects.go

@@ -0,0 +1,31 @@
+package model
+
+import "strings"
+
+// InjectProjectNodes adds dynamic project branches under projects.dynamic.
+func (t *NavTree) InjectProjectNodes(projects []Item) {
+	// Drop prior dynamic project.* nodes.
+	for id := range t.nodes {
+		if strings.HasPrefix(id, "project.") {
+			delete(t.nodes, id)
+		}
+	}
+	t.children["projects.dynamic"] = nil
+	for _, p := range projects {
+		pid := "project." + p.ID
+		t.nodes[pid] = NavNode{ID: pid, Label: p.Title, Branch: true, ParentID: "projects.dynamic"}
+		t.children["projects.dynamic"] = append(t.children["projects.dynamic"], pid)
+		taskID := pid + ".tasks"
+		fileID := pid + ".files"
+		t.nodes[taskID] = NavNode{
+			ID: taskID, Label: "Tasks", Branch: false, ParentID: pid,
+			List: &ListSpec{Subject: SubjectTasks, ProjectID: p.ID},
+		}
+		t.nodes[fileID] = NavNode{
+			ID: fileID, Label: "Files", Branch: false, ParentID: pid,
+			List: &ListSpec{Subject: SubjectProjectFiles, ProjectID: p.ID},
+		}
+		t.children[pid] = []string{taskID, fileID}
+	}
+	t.rebuildVisible()
+}