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_test.go

@@ -0,0 +1,75 @@
+package model_test
+
+import (
+	"testing"
+
+	"github.com/eslider/go-onlyoffice/cmd/office/model"
+)
+
+func TestNavTreeHasExpectedRoots(t *testing.T) {
+	tree := model.DefaultNavTree()
+	roots := tree.RootLabels()
+	want := []string{"Projects", "Calendar", "CRM", "Mail", "Users"}
+	if len(roots) != len(want) {
+		t.Fatalf("roots=%v want %v", roots, want)
+	}
+	for i, w := range want {
+		if roots[i] != w {
+			t.Fatalf("root[%d]=%q want %q", i, roots[i], w)
+		}
+	}
+}
+
+func TestNavExpandCollapse(t *testing.T) {
+	tree := model.DefaultNavTree()
+	if !tree.IsExpandable(0) {
+		t.Fatal("Projects should be expandable")
+	}
+	tree.ToggleExpand(0)
+	if !tree.IsExpanded(0) {
+		t.Fatal("Projects should expand")
+	}
+	tree.ToggleExpand(0)
+	if tree.IsExpanded(0) {
+		t.Fatal("Projects should collapse")
+	}
+}
+
+func TestNavLeafReturnsListSpec(t *testing.T) {
+	tree := model.DefaultNavTree()
+	tree.ToggleExpand(0) // Projects
+	tree.ToggleExpand(1) // Browse
+	// Find "All projects" leaf cursor
+	var found int = -1
+	for i := 0; i < tree.VisibleCount(); i++ {
+		n, ok := tree.NodeAtVisible(i)
+		if ok && n.List != nil && n.List.Subject == model.SubjectProjects {
+			found = i
+			break
+		}
+	}
+	if found < 0 {
+		t.Fatal("all projects leaf not found")
+	}
+	tree.SetCursor(found)
+	spec, ok := tree.CurrentListSpec()
+	if !ok || spec.Subject != model.SubjectProjects {
+		t.Fatalf("spec=%v ok=%v", spec, ok)
+	}
+}
+
+func TestPrevFocusPane(t *testing.T) {
+	if got := model.PrevFocusPane(model.FocusList); got != model.FocusMenu {
+		t.Fatalf("got %v", got)
+	}
+	if got := model.PrevFocusPane(model.FocusMenu); got != model.FocusPreview {
+		t.Fatalf("got %v", got)
+	}
+}
+
+func TestActionsForContact(t *testing.T) {
+	acts := model.ActionsFor(model.KindContact)
+	if len(acts) < 2 {
+		t.Fatalf("expected view+delete, got %d", len(acts))
+	}
+}