Add frontend testing

We have solid coverage on models and controllers, but nothing that actually drives the app in a browser. Anything that only breaks in the browser (forms, Turbo frames, Stimulus controllers, multi-step flows) has to be caught by someone clicking through it manually, which means it usually gets caught after it ships.

I would recommend Capybara. Rails already bundles it for system tests. `ActionDispatch::SystemTestCase` gives us a headless browser and it runs alongside what we have.

It also gives Claude Code a way to test whatever is generated

Start by reviewing the existing model and controller test setup, then read Rails system-test documentation for Capybara and ActionDispatch::SystemTestCase. Define which browser flows should be covered first, including forms, Turbo frames, Stimulus controllers, or multi-step flows. Done means the project has a usable headless-browser testing approach and representative frontend coverage.

Written by the indexing model from the issue text.