Fix js-dos canvas centering

frontend/src/pages/Play.svelte

@@ -73,6 +73,7 @@
       dosCI = window.Dos(dosContainer, {
         url,
         autoSave: true,
+        renderAspect: "4/3",
         // Server-backed FS persistence: in-game saves survive sessions and
         // follow the player across browsers/devices. Both normal and setup
         // mode share one save set so SETUP.EXE changes carry over.
@@ -253,10 +254,6 @@
   @media (max-width: 640px) {
     .dos-container { aspect-ratio: 4/3; max-height: 50vh; border-radius: var(--radius-sm); }
   }
-  .dos-container :global(canvas) {
-    width: 100% !important;
-    height: 100% !important;
-  }
 
   /* Overlay shown while loading / booting, covers the empty container */
   .overlay {

frontend/src/pages/__tests__/Play.test.js

@@ -97,6 +97,7 @@ describe("Play", () => {
       const opts = dosMock.mock.calls[0][1];
       expect(opts.url).toBe("/games/game-1.jsdos");
       expect(opts.autoSave).toBe(true);
+      expect(opts.renderAspect).toBe("4/3");
 
       // Both normal and setup bundles map to the same per-game save key
       await expect(opts.fsChanges.urlToKey("/games/game-1.jsdos")).resolves.toBe("game-1");