fix: use stable unique keys for quest reward list

The previous key `${reward.type}-${amount ?? ""}` collapsed to
"adventurer-" for every adventurer-unlock reward (which carries no
amount), producing duplicate-key React warnings on every render tick.
Because console.error is forwarded to the backend telemetry service,
this caused continuous email alerts.

The key now uses targetId (present on adventurer and upgrade rewards)
first, falls back to amount (present on gold/essence/crystal rewards),
and uses the map index only as a last resort.

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

@@ -108,9 +108,9 @@ const QuestCard = ({
           </p>
         }
         <div className="quest-rewards">
-          {quest.rewards.map((reward) => {
+          {quest.rewards.map((reward, rewardIndex) => {
             return (
-              <span className="reward-tag" key={`${reward.type}-${String(reward.amount ?? "")}`}>
+              <span className="reward-tag" key={`${reward.type}-${reward.targetId ?? String(reward.amount ?? rewardIndex)}`}>
                 {reward.type === "gold"
                   && `🪙 ${formatNumber(reward.amount ?? 0)}`}
                 {reward.type === "essence"