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.go

@@ -39,8 +39,19 @@ const (
 	SubjectProjectFiles  Subject = "project_files"
 	SubjectTaskFiles     Subject = "task_files"
 	SubjectUsers         Subject = "users"
+	SubjectCalendar      Subject = "calendar"
 )
 
+// SubjectIsMail reports whether a list subject loads mailbox messages.
+func SubjectIsMail(s Subject) bool {
+	switch s {
+	case SubjectMailInbox, SubjectMailSent, SubjectMailDrafts, SubjectMailTrash, SubjectMailSpam:
+		return true
+	default:
+		return false
+	}
+}
+
 // FocusPane is which column has keyboard focus.
 type FocusPane int