fix: properly detect Supabase error responses in shop/achievements APIs

Check status code and absence of error 'code' key to distinguish
real data arrays from PostgREST error objects.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

api/achievements.php

@@ -19,7 +19,7 @@ if ($category) {
 }
 
 $achRes = supabase_rest('GET', $endpoint, [], $token);
-$achievements = (isset($achRes['data']) && is_array($achRes['data'])) ? $achRes['data'] : [];
+$achievements = ($achRes['status'] === 200 && is_array($achRes['data']) && !isset($achRes['data']['code'])) ? $achRes['data'] : [];
 
 if (!empty($achievements)) {
     $unlockedRes = supabase_rest('GET', 'player_achievements?select=achievement_id,unlocked_at', [], $token);

api/shop.php

@@ -17,7 +17,7 @@ if ($method === 'GET') {
     $category = $_GET['category'] ?? 'boards';
 
     $res = supabase_rest('GET', "shop_items?category=eq.{$category}&select=*&order=price.asc", [], $token);
-    $items = (isset($res['data']) && is_array($res['data'])) ? $res['data'] : [];
+    $items = ($res['status'] === 200 && is_array($res['data']) && !isset($res['data']['code'])) ? $res['data'] : [];
 
     if (!empty($items)) {
         $ownedRes = supabase_rest('GET', 'user_items?select=item_id', [], $token);