Change collapsed sidebar state computation

Fixes a bug where the collapsed value was being computed serverside and
causing the sidebar to be expanded by default on mobile.

Related to Issue #44

pages/marathon/_marathon.vue

@@ -16,8 +16,7 @@ export default Vue.extend({
   data() {
     return {
       marathonId: this.$route.params.marathon,
-      // Show the sidebar by default on desktop-class devices
-      collapsed: (globalThis.innerWidth ?? 1024) < 1024,
+      collapsed: false,
     };
   },
 
@@ -42,6 +41,11 @@ export default Vue.extend({
     },
   },
 
+  mounted(): void {
+    // Show the sidebar by default on desktop-class devices
+    this.collapsed = (globalThis.innerWidth ?? 1024) < 1024;
+  },
+
   watch: {
     $route(to, from): void {
       // Detect when the path changes (not anchors in the same page or queries)