Add dark mode toggle button

Persists preference in LocalStorage for logged-out users and updates the preference for logged-in users.

app/controllers/users_controller.rb

 class UsersController < ApplicationController
+  before_action :authenticate_user!, only: [ :update_theme ]
+
   def leaderboard
     @workload_data = User.leaderboard
   end
+
+  def update_theme
+    if current_user.present?
+      # Ensure darkmode parameter is valid (light, dark)
+      theme = params[:darkmode].to_s
+      if [ "light", "dark" ].include?(theme)
+        if current_user.update(darkmode: theme)
+          render json: { success: true, theme: theme }, status: :ok
+        else
+          render json: { success: false, errors: current_user.errors.full_messages }, status: :unprocessable_entity
+        end
+      else
+        render json: { success: false, error: "Invalid theme value" }, status: :bad_request
+      end
+    else
+      render json: { success: false, error: "User not authenticated" }, status: :unauthorized
+    end
+  end
 end