Add Christmas theme and snow effects for December

Introduces a Christmas theme that activates in December, including themed colors, subtle background patterns, and snowflake effects on loading screens. Adds a reusable SnowLoader component and utility for December detection. Updates layout and loading components to conditionally apply decorations and styles only during December.

lib/utils/christmas.ts

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