Drop the dummy app's tooling for asset modes that no longer exist

#3751 removed the :webpacker, :vite and :importmap asset sources but
left their plumbing in the dummy app. None of it is reachable now:

- link_tree ../../../../../src in the Sprockets manifest exposed the
  gem's ES module sources so the importmap pins could resolve them as
  assets. The engine no longer puts src/ on the asset path, so the line
  links nothing; dropping it changes only the manifest's own digest. It
  reached outside the image during the Docker build too, which is what
  the sed alongside it was stripping.
- The FontAwesome path in config/initializers/assets.rb fed the webfonts
  from node_modules to the importmap and webpack builds. The prebuilt
  bundle carries them.
- vite.config.ts, postcss.config.js, babel.config.js and .browserslistrc
  configure bundlers the dummy app no longer runs. webpack.config.js was
  not merely idle: the installer reads its presence as :external, so
  auto-detection inside the dummy app resolved to the wrong pipeline.
- app/javascript/application.js imports packages that are no longer
  dependencies, and host-side bundling is already covered by the
  :external mode's app/frontend entrypoints.
- vendor/javascript is where importmap-rails vendors its pins.
- Procfile.dev ran a build:css script and bin/vite, neither of which
  exists.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: mshibuya

spec/dummy_app/.browserslistrc

@@ -20,11 +20,4 @@
 /public/system
 
 /public/assets
-/public/packs
-/public/packs-test
-/public/vite*
 /node_modules
-/yarn-error.log
-/yarn.lock
-yarn-debug.log*
-.yarn-integrity

spec/dummy_app/.gitignore

@@ -51,7 +51,6 @@ RUN bundle install && \
 # RUN npm install
 
 # Precompiling assets for production without requiring secret RAILS_MASTER_KEY
-RUN sed -i "/link_tree ..\/..\/..\//d" app/assets/config/manifest.js
 RUN SECRET_KEY_BASE=DUMMY ./bin/rails assets:precompile db:setup
 
 # Final stage for app image

spec/dummy_app/Dockerfile

@@ -1,3 +1 @@
 web: bin/rails server -p 3000
-css: yarn build:css --watch
-vite: bin/vite dev

spec/dummy_app/Procfile.dev

@@ -1,4 +1,3 @@
 //= link_tree ../images
-//= link_tree ../../../../../src .js
 //= link turbo.js
 //= link application.css

spec/dummy_app/app/assets/config/manifest.js

@@ -7,7 +7,6 @@
 
 # Add additional assets to the asset load path
 # Rails.application.config.assets.paths << Emoji.images_path
-Rails.application.config.assets.paths << Rails.root.join('node_modules/@fortawesome/fontawesome-free/webfonts') if Rails.application.config.respond_to?(:assets)
 
 # Precompile additional assets.
 # application.js, application.css, and all non-JS/CSS in app/assets folder are already added.