Add HasChanges trait with impl_has_changes! macro, validate_update()
and update_response() helpers to reduce boilerplate across all 7 entity
update handlers. Extract edit form actions (error, spinner, buttons)
into a shared Askama macro. Also adds missing no-changes validation
to the bag update handler.
Cover text fields, numeric fields, +/- adjuster buttons, searchable
selects, image upload/replace/remove, deferred image save, and the
cancel button across roaster, gear, cafe, brew, and cup entities.
Add wait_for_url_not_contains helper to correctly detect post-submit
redirects when the edit page URL shares a prefix with the detail page.
