add no-JS fallback for provider forms

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>

lib/berrypod_web/router.ex

@@ -144,6 +144,8 @@ defmodule BerrypodWeb.Router do
     post "/settings/from-address", SettingsController, :update_from_address
     post "/settings/stripe/signing-secret", SettingsController, :update_signing_secret
     post "/navigation", NavigationController, :save
+    post "/providers", ProvidersController, :create
+    post "/providers/:id", ProvidersController, :update
 
     live_session :admin,
       layout: {BerrypodWeb.Layouts, :admin},