Splits article_row.rs and subscribe_form.rs into mod.rs + component.rs +
handlers.rs (mirroring the components/*/ convention), and adds a sibling
app/handlers.rs for App's heading computation. RSX now calls into plain
functions (score_percent, normalize_feed_url, subscribed_message,
feed_heading) that are covered by unit tests, instead of computing the
same logic inline where it can't be tested without a Dioxus runtime.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
