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.

packages/types/src/interfaces/Quest.ts

@@ -25,4 +25,6 @@ export interface Quest {
   zoneId: string;
   /** Minimum party combat power required to start this quest */
   combatPowerRequired?: number;
+  /** Unix timestamp of the most recent failed attempt (if any) */
+  lastFailedAt?: number;
 }