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>

server/src/Hokm.Server/Profiles/ProfileService.cs

@@ -131,6 +131,7 @@ public class ProfileService
             if (p.Coins < pk.Price) return (false, p, "insufficient");
             p.Coins -= pk.Price;
             Gamification.GrantXp(p, pk.Xp);
+            Gamification.EvaluateAchievements(p); // unlock any level milestones reached
             await Save(p);
             await Ledger(uid, "xp", -pk.Price, id);
             return (true, p, "");