Fix Chrome Android viewport overflow with position fixed game screen

Use position:fixed with inset:0 for the game screen instead of
height-based sizing. This bypasses the Chrome Android 100vh bug where
vh includes space behind the dynamic URL bar. Also adds
-webkit-fill-available fallback on body.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/style.css

@@ -4907,6 +4907,7 @@ body.screen-shake {
 
 body.mobile-portrait {
     height: 100vh;
+    height: -webkit-fill-available;
     height: 100dvh;
     overflow: hidden;
     overscroll-behavior: contain;
@@ -4915,23 +4916,19 @@ body.mobile-portrait {
 
 body.mobile-portrait #app {
     padding: 0;
-    height: 100vh;
-    height: 100dvh;
-    max-height: 100vh;
-    max-height: 100dvh;
+    height: 100%;
     overflow: hidden;
 }
 
 /* --- Mobile: Game screen fills viewport --- */
 /* IMPORTANT: Must include .active to avoid overriding .screen { display: none } */
 body.mobile-portrait #game-screen.active {
-    height: 100vh;
-    height: 100dvh;
-    max-height: 100vh;
-    max-height: 100dvh;
+    position: fixed;
+    top: 0;
+    left: 0;
+    right: 0;
+    bottom: 0;
     overflow: hidden;
-    margin-left: 0;
-    width: 100%;
     display: flex;
     flex-direction: column;
 }