fix: preserve achievements across prestige

Fixes #38. buildPostPrestigeState was using structuredClone(defaultAchievements)
via the freshState, which reset all achievements on every prestige. Achievements
are now carried forward from currentState.achievements instead, ensuring unlocked
achievements are never lost across prestige resets.

apps/api/src/services/prestige.ts

@@ -225,7 +225,9 @@ const buildPostPrestigeState = (
 
   const prestigeState: GameState = {
     ...freshState,
-    lastTickAt: Date.now(),
+    // Achievements are permanent — earned achievements survive all prestiges
+    achievements: currentState.achievements,
+    lastTickAt:   Date.now(),
 
     /*
      * Fold current-run totals into lifetime stats so the GameState reflects