wasm: center non-resizable UIs

Signed-off-by: falkTX <falktx@falktx.com>

utils/emscripten.html.in

@@ -32,6 +32,12 @@
         transform: scale(calc(1 / var(--device-pixel-ratio)));
         width: 100vw;
         height: 100vh;
+        /* center */
+        margin: auto auto;
+        top: 0;
+        bottom: 0;
+        left: 0;
+        right: 0;
       }
 
       #error {