feat(office): mail preview, infinite scroll, pane resize, and scrollbars

Polish the office TUI with proportional pane layout, drag resize, live
filtering, HTML mail preview, paginated mail lists, overflow scrollbars,
flattened navigation, and project save/status fixes; add UpdateProjectStatus.

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

cmd/office/ui/detail_scroll_test.go

@@ -0,0 +1,60 @@
+package ui
+
+import (
+	"testing"
+
+	tea "github.com/charmbracelet/bubbletea"
+	"github.com/eslider/go-onlyoffice/cmd/office/model"
+)
+
+func TestDetailDocumentScrollByKey(t *testing.T) {
+	d := newDetailPane()
+	d.SetSize(50, 20)
+	longBody := "# Mail\n\n"
+	for i := 0; i < 80; i++ {
+		longBody += "line\n"
+	}
+	d.LoadDocument(model.Item{ID: "1", Kind: model.KindMail, Title: "M"}, longBody, 40)
+	d.SetFocused(true)
+
+	if d.DocumentYOffset() != 0 {
+		t.Fatalf("expected top offset 0, got %d", d.DocumentYOffset())
+	}
+	if !d.ScrollDocument("down") {
+		t.Fatal("expected scroll down to succeed")
+	}
+	if d.DocumentYOffset() == 0 {
+		t.Fatal("expected offset after scroll down")
+	}
+	if !d.ScrollDocument("up") {
+		t.Fatal("expected scroll up to succeed")
+	}
+}
+
+func TestDetailDocumentMouseWheel(t *testing.T) {
+	d := newDetailPane()
+	d.SetSize(50, 20)
+	longBody := "# Mail\n\n"
+	for i := 0; i < 80; i++ {
+		longBody += "line\n"
+	}
+	d.LoadDocument(model.Item{ID: "1", Kind: model.KindMail, Title: "M"}, longBody, 40)
+
+	d.ScrollDocumentMouse(tea.MouseMsg{
+		X: 10, Y: 2, Button: tea.MouseButtonWheelDown, Action: tea.MouseActionPress,
+	})
+	if d.DocumentYOffset() == 0 {
+		t.Fatal("expected wheel down to scroll document")
+	}
+}
+
+func TestDetailPaneXRange(t *testing.T) {
+	pw := PaneWidths{
+		Menu: 12, List: 72, Detail: 36,
+		Visibility: PaneVisibility{Menu: true, List: true, Detail: true},
+	}
+	start, end := DetailPaneXRange(pw)
+	if start != 84 || end != 120 {
+		t.Fatalf("range=%d..%d want 84..120", start, end)
+	}
+}