feat: add server-side amount validation in membership fee type form

- Validate amount format on input change
- Clean invalid characters from amount input
- Provides immediate feedback on invalid input

lib/mv_web/live/membership_fee_type_live/form.ex

@@ -210,6 +210,9 @@ defmodule MvWeb.MembershipFeeTypeLive.Form do
 
   @impl true
   def handle_event("validate", %{"membership_fee_type" => params}, socket) do
+    # Validate amount format if present
+    params = validate_amount_format(params)
+
     # Merge with existing form values to preserve unchanged fields
     # Extract values directly from form fields to get current state
     existing_values = get_existing_form_values(socket.assigns.form)