feat: add join form

lib/mv_web/router.ex

@@ -15,6 +15,7 @@ defmodule MvWeb.Router do
     plug :load_from_session
     plug :set_locale
     plug MvWeb.Plugs.CheckPagePermission
+    plug MvWeb.Plugs.JoinFormEnabled
   end
 
   pipeline :api do
@@ -126,6 +127,12 @@ defmodule MvWeb.Router do
       overrides: [MvWeb.AuthOverrides, AshAuthentication.Phoenix.Overrides.DaisyUI],
       gettext_backend: {MvWeb.Gettext, "auth"}
 
+    # Public join page (no auth required)
+    live_session :public_join,
+      on_mount: [{MvWeb.LiveUserAuth, :live_user_optional}] do
+      live "/join", JoinLive, :index
+    end
+
     # Public join confirmation (double opt-in); /confirm* is already public in CheckPagePermission
     get "/confirm_join/:token", JoinConfirmController, :confirm