fix: 베팅과 게임 정보 창을 화면 중앙에 배치한다

공통 viewport 중앙 배치 규칙을 추가하고 데스크톱·모바일 Chromium 좌표 회귀 검사를 보강한다.

app/game-frontend/src/assets/main.css

@@ -64,6 +64,17 @@ textarea {
     font: inherit;
 }
 
+/*
+ * Tailwind's reset removes the native dialog's auto margins. Dialogs that are
+ * meant to float over the current screen opt back into explicit viewport
+ * centring instead of inheriting the document's 500px legacy canvas.
+ */
+.viewport-centered-dialog {
+    position: fixed;
+    inset: 0;
+    margin: auto;
+}
+
 /*
  * Firefox for Android may zoom to a focused search field. `manipulation`
  * suppresses that focus-only zoom while retaining ordinary pan and pinch zoom.