feat: default to the new mobile layout

Make the new mobile layout the default when no preference is stored, and
relabel the legacy 'default' option as Old (localized as Previous/older where
it reads better) while keeping its stored value for backwards compatibility.

packages/ui/src/lib/mobileLayoutPreference.ts

@@ -3,18 +3,20 @@ export type MobileLayoutPreference = 'default' | 'new';
 const MOBILE_LAYOUT_PREFERENCE_KEY = 'openchamber-mobile-layout';
 
 export const normalizeMobileLayoutPreference = (value: unknown): MobileLayoutPreference => {
-  return value === 'new' ? 'new' : 'default';
+  // 'new' is the default; only an explicit 'default' (the legacy/"Old" layout)
+  // opts out of it.
+  return value === 'default' ? 'default' : 'new';
 };
 
 export const getStoredMobileLayoutPreference = (): MobileLayoutPreference => {
   if (typeof window === 'undefined') {
-    return 'default';
+    return 'new';
   }
 
   try {
     return normalizeMobileLayoutPreference(window.localStorage.getItem(MOBILE_LAYOUT_PREFERENCE_KEY));
   } catch {
-    return 'default';
+    return 'new';
   }
 };