feat: implement dark mode support

[camilleislasse]

Summary

Add dark mode support to the BootstrapAdminUi bundle using Bootstrap 5.3 color modes.

Features

• Theme toggle button (sun/moon) in the navbar
• Persistent preference via localStorage with prefers-color-scheme fallback
• Anti-flash inline script applied before page paint
• Login page logo adapts to active theme
• SCSS refactored to use CSS custom properties (--tblr-*) throughout
• ApexCharts re-renders on theme change via MutationObserver

Changes

• Add theme-switcher.js script
• Add theme.html.twig anti-flash hookable in #stylesheets (priority 200)
• Add theme_switcher.html.twig navbar button hookable
• Refactor _body.scss, _sidebar.scss, _tom-select.scss with include color-mode() mixin
• Replace hardcoded colors in _accordion.scss, _filters.scss, _form.scss, _loader.scss, _navbar.scss
• Update statistics_chart.js to use CSS vars and re-render on theme change
• Update login logo template to support both light/dark variants