fix: 바텀시트가 하단 네비에 가려지는 문제 — z-index 1010 + padding-bottom 확대

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

system1-factory/web/css/purchase-mobile.css

@@ -105,7 +105,7 @@
   position: fixed;
   inset: 0;
   background: rgba(0,0,0,0.4);
-  z-index: 100;
+  z-index: 1005;
   opacity: 0;
   pointer-events: none;
   transition: opacity 0.25s;
@@ -119,12 +119,12 @@
   right: 0;
   background: white;
   border-radius: 16px 16px 0 0;
-  z-index: 101;
+  z-index: 1010;
   max-height: 92vh;
   overflow-y: auto;
   transform: translateY(100%);
   transition: transform 0.3s cubic-bezier(0.4, 0, 0.2, 1);
-  padding-bottom: env(safe-area-inset-bottom);
+  padding-bottom: calc(20px + env(safe-area-inset-bottom));
 }
 .pm-sheet.open { transform: translateY(0); }
 .pm-sheet-handle {