feat: add custom field slug

lib/mv_web/live/custom_field_live/form.ex

@@ -19,6 +19,9 @@ defmodule MvWeb.CustomFieldLive.Form do
   - immutable - If true, values cannot be changed after creation (default: false)
   - required - If true, all members must have this custom field (default: false)
 
+  **Read-only (Edit mode only):**
+  - slug - Auto-generated URL-friendly identifier (immutable)
+
   ## Value Type Selection
   - `:string` - Text data (unlimited length)
   - `:integer` - Numeric data
@@ -48,6 +51,20 @@ defmodule MvWeb.CustomFieldLive.Form do
 
       <.form for={@form} id="custom_field-form" phx-change="validate" phx-submit="save">
         <.input field={@form[:name]} type="text" label={gettext("Name")} />
+
+        <%!-- Show slug in edit mode (read-only) --%>
+        <div :if={@custom_field} class="mb-4">
+          <label class="block text-sm font-semibold leading-6 text-zinc-800">
+            {gettext("Slug")}
+          </label>
+          <div class="mt-2 p-3 bg-zinc-50 border border-zinc-300 rounded-lg text-sm text-zinc-700">
+            {@custom_field.slug}
+          </div>
+          <p class="mt-2 text-sm leading-6 text-zinc-600">
+            {gettext("Auto-generated identifier (immutable)")}
+          </p>
+        </div>
+
         <.input
           field={@form[:value_type]}
           type="select"