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>

dev/docker-compose.yml

@@ -8,6 +8,7 @@ services:
       - redmine-files:/usr/src/redmine/files
       - ../tonalmine:/usr/src/redmine/themes/tonalmine
       - ./plugins:/usr/src/redmine/plugins
+      - ../redmine_tonalmine:/usr/src/redmine/plugins/redmine_tonalmine
       - .:/seed:ro
 
 volumes: