Disable Christmas theme and improve layout skeleton

Christmas decorations and theme logic have been disabled throughout the app, including the isDecember utility, layout, and related imports. Layout now shows a skeleton UI while mounting to prevent layout shift. Minor improvements to RevenueChart tooltip colors and ChatDetail request headers for better consistency.

components/dashboard/ChatDetail.tsx

@@ -473,7 +473,7 @@ export default function ChatDetail({ chatId }: { chatId: string }) {
             content: newMessage 
           }),
           headers: {
-            'Content-Type': 'application/json'
+            'Content-Type': 'application/json; charset=utf-8'
           }
         });
       }