UX: Splash should always stick to top left corner of the viewport (#17348)

No visual changes.

This commit only fixes some display issues on some browsers that don't respect the defaults.

app/views/common/_discourse_splash.html.erb

@@ -11,6 +11,8 @@
       backface-visibility: hidden;
       background: var(--secondary);
       position: absolute;
+      left: 0;
+      top: 0;
       width: 100%;
       z-index: 1001;
       --animation-state: paused;