Update dependencies for rails, stimulus, and tailwindcss

[luciengeorge]

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Upgrade Rails from 6.1 to 8.0, TailwindCSS from 2.1 to 4.0, and modernize the asset pipeline to leverage latest features and improve performance.

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The previous setup used Webpacker and older versions of Rails and TailwindCSS, leading to a complex build process and missing out on significant performance and feature improvements. This PR migrates to Rails 8's native asset pipeline (Importmap/Sprockets), introduces Solid Queue and Solid Cache for simplified background jobs and caching, and updates TailwindCSS to its standalone CLI for much faster CSS builds.