feat: redmine_tonalmine plugin for default and per-account appearance

Admins set the default mode, contrast and color on Settings > Display,
shown as three labelled form rows below the Theme select. Logged-in
users' choices are saved on their account and override only the parts
they changed; anonymous visitors get the default.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

redmine_tonalmine/app/controllers/tonalmine_appearance_controller.rb

@@ -0,0 +1,17 @@
+# Saves the current user's appearance (sent by theme.js). The body replaces the whole saved
+# choice; an empty object resets the user to the default.
+class TonalmineAppearanceController < ApplicationController
+  before_action :require_login
+
+  def update
+    prefs = params.to_unsafe_h.except('controller', 'action', 'tonalmine_appearance')
+    errors = RedmineTonalmine::Appearance.errors(prefs)
+    if errors.any?
+      render json: {errors: errors}, status: :unprocessable_content
+    else
+      User.current.pref[:tonalmine] = RedmineTonalmine::Appearance.clean(prefs)
+      User.current.pref.save
+      head :no_content
+    end
+  end
+end