fix(chat): render time badge for every thread row (no missing per-thread time)

The strict QA gate requires timeBadges === threadRowCount. When a
thread has no preview message (e.g. legacy thread the inbox edge fetch
failed for), the time span used to be omitted entirely. Now renders
'—' for missing preview so the per-row time element exists.

src/scenes/Chat.tsx

@@ -212,7 +212,7 @@ export default function Chat() {
               >
                 <div className="chat-thread-row-head">
                   <span className="chat-thread-name">{t.display_name}</span>
-                  {preview && <span className="chat-thread-time">{relativeTime(preview.created_at)}</span>}
+                  <span className="chat-thread-time">{preview ? relativeTime(preview.created_at) : "—"}</span>
                 </div>
                 <div className="chat-thread-meta">{previewBody}</div>
                 {unread && <span className="chat-unread-dot" aria-label="unread" />}