Upgrade Rails to 7.1 and localize legacy Bootstrap assets

[parterburn]

Summary

• upgrade the app from Rails 6.1 to Rails 7.1.6 while keeping the existing config.load_defaults 6.1 behavior for a smaller runtime shift
• replace the incompatible twbs_sass_rails dependency by vendoring its Bootstrap 3 / Font Awesome asset tree into vendor/assets
• switch from sass-rails to sassc-rails and add the explicit respond.js manifest link required by the newer Sprockets behavior
• update the Rails test harness for Rails 7.1 by moving to database_cleaner-active_record
• fix ApplicationController#redirect_back_or_to to match the Rails 7 helper signature used by Devise account updates

Testing

• bundle exec rails runner "puts \"rails=#{Rails.version}\"; puts \"rack=#{Rack.release}\"; puts \"env=#{Rails.env}\""
• SECRET_KEY_BASE_DUMMY=1 RAILS_ENV=production bundle exec rails assets:precompile
• bundle audit check --update
• bundle exec rspec spec/controllers/registrations_controller_spec.rb:19 spec/controllers/registrations_controller_spec.rb:26
• bundle exec rspec spec/controllers/welcome_controller_spec.rb spec/features/welcome_spec.rb
• bundle exec rspec spec/controllers/registrations_controller_spec.rb:117 spec/controllers/registrations_controller_spec.rb:130 spec/controllers/registrations_controller_spec.rb:143 spec/controllers/registrations_controller_spec.rb:155
• manual browser walkthrough of / and /settings/u7vtaqn27izeyvo6999

Follow-ups

• Rails 7.1 deprecation warnings remain around Rails.application.secrets, mailer preview path configuration, cache format version, and some test-only configuration
• unrelated pre-existing db/schema.rb changes remain outside this PR