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/live/admin/providers/form.html.heex

@@ -27,7 +27,18 @@
     </div>
   <% end %>
 
-  <.form for={@form} id="provider-form" phx-change="validate" phx-submit="save">
+  <.form
+    for={@form}
+    id="provider-form"
+    action={
+      if @live_action == :new,
+        do: ~p"/admin/providers",
+        else: ~p"/admin/providers/#{@connection.id}"
+    }
+    method="post"
+    phx-change="validate"
+    phx-submit="save"
+  >
     <input type="hidden" name="provider_connection[provider_type]" value={@provider_type} />
 
     <.input