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/engine/tick.ts

@@ -46,6 +46,32 @@ const checkAchievements = (state: GameState): Achievement[] => {
 /** Maximum value any resource can accumulate to. Beyond this JS floats lose all useful precision. */
 export const RESOURCE_CAP = 1e300;
 
+/**
+ * Probability of quest failure per zone — scales from 10% (early game) to 40% (end game).
+ * On failure the quest resets to "available" with no rewards; the player must wait the
+ * full duration again on their next attempt.
+ */
+const ZONE_FAILURE_CHANCE: Record<string, number> = {
+  verdant_vale: 0.10,
+  shattered_ruins: 0.12,
+  frozen_peaks: 0.14,
+  shadow_marshes: 0.16,
+  volcanic_depths: 0.18,
+  astral_void: 0.20,
+  celestial_reaches: 0.22,
+  abyssal_trench: 0.24,
+  infernal_court: 0.26,
+  crystalline_spire: 0.28,
+  void_sanctum: 0.30,
+  eternal_throne: 0.32,
+  primordial_chaos: 0.34,
+  infinite_expanse: 0.36,
+  reality_forge: 0.38,
+  cosmic_maelstrom: 0.40,
+  primeval_sanctum: 0.40,
+  the_absolute: 0.40,
+};
+
 const capResource = (value: number): number => Math.min(value, RESOURCE_CAP);
 
 /**
@@ -128,6 +154,11 @@ export const applyTick = (state: GameState, deltaSeconds: number): GameState =>
       return quest;
     }
 
+    const failureChance = ZONE_FAILURE_CHANCE[quest.zoneId] ?? 0.20;
+    if (Math.random() < failureChance) {
+      return { ...quest, status: "available" as const, startedAt: undefined, lastFailedAt: now };
+    }
+
     for (const reward of quest.rewards) {
       if (reward.type === "gold" && reward.amount != null) {
         questGold += reward.amount;