Modernize examples app with Tailwind

[vipulnsward]

Summary

• switch Uploadcare dependencies to the published 5.0.0.rc1 prerelease gems
• add Rails-native Tailwind CSS and replace the Bootstrap/jQuery shell with Tailwind layout partials
• refresh the examples overview, sidebar, flashes, and common state-changing actions for Hotwire-friendly Rails patterns
• update README and examples coverage for the new shell

Verification

• bin/ci passes locally: bundle install, pnpm install/build, tailwindcss:build, rubocop, rails_best_practices, brakeman, rspec
• rspec: 122 examples, 0 failures
• browser smoke checked /examples at desktop and mobile viewport sizes