fix: handle form errors correctly in membership fee settings

- Fix Protocol.UndefinedError when iterating over form errors
- Handle both tuple and list error formats
- Prevents crash when saving settings with validation errors

lib/mv_web/live/membership_fee_settings_live.ex

@@ -101,9 +101,12 @@ defmodule MvWeb.MembershipFeeSettingsLive do
                     )})
                   </option>
                 </select>
-                <%= for {msg, _opts} <- @form.errors[:default_membership_fee_type_id] || [] do %>
+                <%= if @form.errors[:default_membership_fee_type_id] do %>
+                  <%= for error <- List.wrap(@form.errors[:default_membership_fee_type_id]) do %>
+                    <% {msg, _opts} = if is_tuple(error), do: error, else: {error, []} %>
                     <p class="text-error text-sm mt-1">{msg}</p>
                   <% end %>
+                <% end %>
                 <p class="text-sm text-base-content/60 mt-2">
                   {gettext(
                     "This membership fee type is automatically assigned to all new members. Can be changed individually per member."
@@ -125,9 +128,12 @@ defmodule MvWeb.MembershipFeeSettingsLive do
                     {gettext("Include joining cycle")}
                   </span>
                 </label>
-                <%= for {msg, _opts} <- @form.errors[:include_joining_cycle] || [] do %>
+                <%= if @form.errors[:include_joining_cycle] do %>
+                  <%= for error <- List.wrap(@form.errors[:include_joining_cycle]) do %>
+                    <% {msg, _opts} = if is_tuple(error), do: error, else: {error, []} %>
                     <p class="text-error text-sm ml-9 mt-1">{msg}</p>
                   <% end %>
+                <% end %>
                 <div class="ml-9 space-y-2">
                   <p class="text-sm text-base-content/60">
                     {gettext("When active: Members pay from the cycle of their joining.")}