feat(office): add Workspace TUI with shared bootstrap and test suite

Introduce the office Bubble Tea browser (module tree, multi-select lists,
markdown preview) alongside oo, with cmd/internal/bootstrap for shared auth,
unit tests across preview/model/ui/fetch, CI build for both binaries, and
GoReleaser archives for office.

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

cmd/office/model/item_test.go

@@ -0,0 +1,45 @@
+package model_test
+
+import (
+	"testing"
+
+	"github.com/eslider/go-onlyoffice/cmd/office/model"
+)
+
+func TestSelectionToggle(t *testing.T) {
+	items := []model.Item{
+		{ID: "1", Title: "A"},
+		{ID: "2", Title: "B"},
+	}
+	sel := model.NewSelection()
+	if sel.Count() != 0 {
+		t.Fatalf("initial count=%d", sel.Count())
+	}
+	sel.Toggle(&items, 0)
+	if !items[0].Selected || sel.Count() != 1 {
+		t.Fatalf("after toggle 0: selected=%v count=%d", items[0].Selected, sel.Count())
+	}
+	sel.Toggle(&items, 0)
+	if items[0].Selected || sel.Count() != 0 {
+		t.Fatalf("after second toggle: selected=%v count=%d", items[0].Selected, sel.Count())
+	}
+}
+
+func TestSelectionClearOnSubjectChange(t *testing.T) {
+	items := []model.Item{{ID: "1", Title: "A", Selected: true}}
+	sel := model.NewSelection()
+	sel.Toggle(&items, 0)
+	sel.Clear()
+	if sel.Count() != 0 {
+		t.Fatalf("count=%d after clear", sel.Count())
+	}
+}
+
+func TestNextFocusPane(t *testing.T) {
+	if got := model.NextFocusPane(model.FocusMenu); got != model.FocusList {
+		t.Fatalf("menu->list got %v", got)
+	}
+	if got := model.NextFocusPane(model.FocusPreview); got != model.FocusMenu {
+		t.Fatalf("preview->menu got %v", got)
+	}
+}