Progressive enhancement: provider form now works without JavaScript.
Forms POST to ProvidersController (create/update), which handles
validation and redirects with flash messages.
With JS: LiveView phx-submit handles save, navigates with flash.
Without JS: Form POSTs to controller, redirects with flash.
Completes Task 3 of notification overhaul (admin forms migration).
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
