feat: add auto-prestige toggle via runestone shop upgrade

Adds a new Utility category to the prestige shop with the "Autonomous Ascension" upgrade (100 runestones). Once purchased, a toggle button appears in the shop that enables automatic prestige — the client RAF loop checks the threshold on each tick and fires the prestige API automatically using the current character name.

✨ This feature was implemented with help from Hikari~ 🌸

apps/api/src/data/prestigeUpgrades.ts

@@ -184,6 +184,16 @@ export const DEFAULT_PRESTIGE_UPGRADES: PrestigeUpgrade[] = [
     runestonesCost: 1_200,
     multiplier: 25,
   },
+  // ── Utility Unlocks ───────────────────────────────────────────────────────
+  {
+    id: "auto_prestige",
+    name: "Autonomous Ascension",
+    description:
+      "Unlock the Auto-Prestige toggle. When enabled, you will automatically ascend the moment you reach the prestige threshold — using your current character name.",
+    category: "utility",
+    runestonesCost: 100,
+    multiplier: 1,
+  },
   // ── Runestone Meta-Upgrade ────────────────────────────────────────────────
   {
     id: "runestone_gain_1",

apps/api/src/services/prestige.ts

@@ -101,6 +101,9 @@ export const buildPostPrestigeState = (
     purchasedUpgradeIds,
     lastPrestigedAt: Date.now(),
     ...computeRunestoneMultipliers(purchasedUpgradeIds),
+    ...(currentState.prestige.autoPrestigeEnabled !== undefined
+      ? { autoPrestigeEnabled: currentState.prestige.autoPrestigeEnabled }
+      : {}),
   };
 
   const freshState = INITIAL_GAME_STATE(currentState.player, characterName);