feat: add zone-scaled quest failure chance

Quests now have a random chance of failing when their timer expires.
On failure the quest resets to available (no rewards lost, just time).
Failure chance scales by zone: 10% in Verdant Vale up to 40% in the
end-game zones. The Quest type gains lastFailedAt so the UI can show
a warning on subsequent attempts.

apps/web/src/components/game/QuestPanel.tsx

@@ -60,6 +60,9 @@ const QuestCard = ({ quest, partyCombatPower, unlockHint, zoneHint }: QuestCardP
             {!zoneHint && unlockHint && <p className="unlock-hint">📜 Complete: {unlockHint}</p>}
           </>
         )}
+        {quest.status === "available" && quest.lastFailedAt != null && (
+          <p className="quest-failed-hint">⚠️ Last attempt failed</p>
+        )}
         {quest.status === "available" && (
           <button
             className="start-quest-button"