Celebration animations for purchases, XP gains & achievement unlocks

- New global celebration system: celebration-store (queue) + CelebrationOverlay
  (animated: count-up XP, filling bar, level-up pop, achievement cards; plays
  levelUp/award sounds; tap or auto-dismiss). Rendered in page.tsx.
- Shop: every purchase now celebrates — XP packs animate XP gain + level-up,
  cosmetics show a "purchased!" pop. Newly-unlocked achievements (diffed from
  the profile before/after) animate too.
- XP purchases now actually evaluate achievements: gamification.evaluateAchievements
  (client) + Gamification.EvaluateAchievements (server, called in ShopBuy xp path)
  unlock level milestones + grant their coins.

Verified live: buying XP took L1→L5, unlocked level_5 server-side and credited its
reward. tsc + dotnet + next build clean; images rebuilt :1500/:1505.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/page.tsx

@@ -18,6 +18,7 @@ import { AuthScreen } from "@/components/screens/AuthScreen";
 import { DailyRewardModal } from "@/components/online/DailyRewardModal";
 import { NotificationToaster } from "@/components/online/NotificationToaster";
 import { ResumeGameBar } from "@/components/online/ResumeGameBar";
+import { CelebrationOverlay } from "@/components/online/CelebrationOverlay";
 import { CapacitorBack } from "@/components/CapacitorBack";
 import { useSessionStore } from "@/lib/session-store";
 import { useGameStore } from "@/lib/game-store";
@@ -141,6 +142,7 @@ export default function Page() {
       <DailyRewardModal />
       <NotificationToaster />
       <ResumeGameBar />
+      <CelebrationOverlay />
       <CapacitorBack />
       {loading && null}
     </>