fix: stylesheets not included in feed

[gildesmarais]

This pull request introduces a mechanism to automatically merge global stylesheet configurations into individual feed configs when they do not already define their own stylesheets. This ensures that all feeds can benefit from a consistent global stylesheet unless overridden, and the logic is thoroughly tested to avoid unwanted side effects.

Feed configuration enhancements:

• Added the merge_global_stylesheets method to LocalConfig, which merges global stylesheets into a feed config if the feed does not already specify its own stylesheets. The method duplicates the config to prevent mutation when merging.
• Updated the find method in LocalConfig to use merge_global_stylesheets, ensuring feeds retrieved via .find automatically include global stylesheets when appropriate.
• Incorporated global stylesheet merging into the main feed configuration flow in app.rb, so all feeds processed by the app have global stylesheets merged as needed.

Testing improvements:

• Added comprehensive tests for merge_global_stylesheets in spec/html2rss/web/app/local_config_spec.rb, covering scenarios where configs already have stylesheets, where global stylesheets exist or not, and ensuring config objects are not mutated.
• Added tests for .find to verify that stylesheet merging works as expected when retrieving feed configs.