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.

lib/utils/christmas.ts

@@ -1,9 +1,12 @@
 /**
  * Check if the current date is in December
  * @returns true if current month is December (0-indexed, so 11 = December)
+ * DISABLED: Christmas theme is currently disabled
  */
 export function isDecember(): boolean {
-  const now = new Date();
-  return now.getMonth() === 11; // December is month 11 (0-indexed)
+  // Christmas theme disabled
+  return false;
+  // const now = new Date();
+  // return now.getMonth() === 11; // December is month 11 (0-indexed)
 }