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/model/item_mail_test.go

@@ -0,0 +1,12 @@
+package model
+
+import "testing"
+
+func TestSubjectIsMail(t *testing.T) {
+	if !SubjectIsMail(SubjectMailInbox) {
+		t.Fatal("inbox should be mail")
+	}
+	if SubjectIsMail(SubjectProjects) {
+		t.Fatal("projects should not be mail")
+	}
+}