fix: clear stale pollEndingAt state when starting a new poll

After a countdown-based poll end, pollEndingAt could remain set as a
stale value during the optimistic update in handleStartPolling, causing
the new poll to briefly render in the "Poll Ending" state with 0:00.

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

frontend/src/pages/Picker.jsx

@@ -252,6 +252,8 @@ function Picker() {
     pollStartedAtRef.current = new Date().toISOString();
     setPollActive(true);
     setPollResult(null);
+    setPollEndingAt(null);
+    setShowEndPollOptions(false);
     try {
       await api.post(`/sessions/${activeSession.id}/voting/start`);
     } catch (err) {