Add phx.gen.auth authentication scaffolding

- LiveView-based email/password auth via mix phx.gen.auth
- Auth links removed from public navigation (direct URL access only)
- Accounts context with User schema and token management

app/test/firehose_web/controllers/user_registration_controller_test.exs

@@ -0,0 +1,50 @@
+defmodule FirehoseWeb.UserRegistrationControllerTest do
+  use FirehoseWeb.ConnCase, async: true
+
+  import Firehose.AccountsFixtures
+
+  describe "GET /users/register" do
+    test "renders registration page", %{conn: conn} do
+      conn = get(conn, ~p"/users/register")
+      response = html_response(conn, 200)
+      assert response =~ "Register"
+      assert response =~ ~p"/users/log-in"
+      assert response =~ ~p"/users/register"
+    end
+
+    test "redirects if already logged in", %{conn: conn} do
+      conn = conn |> log_in_user(user_fixture()) |> get(~p"/users/register")
+
+      assert redirected_to(conn) == ~p"/"
+    end
+  end
+
+  describe "POST /users/register" do
+    @tag :capture_log
+    test "creates account but does not log in", %{conn: conn} do
+      email = unique_user_email()
+
+      conn =
+        post(conn, ~p"/users/register", %{
+          "user" => valid_user_attributes(email: email)
+        })
+
+      refute get_session(conn, :user_token)
+      assert redirected_to(conn) == ~p"/users/log-in"
+
+      assert conn.assigns.flash["info"] =~
+               ~r/An email was sent to .*, please access it to confirm your account/
+    end
+
+    test "render errors for invalid data", %{conn: conn} do
+      conn =
+        post(conn, ~p"/users/register", %{
+          "user" => %{"email" => "with spaces"}
+        })
+
+      response = html_response(conn, 200)
+      assert response =~ "Register"
+      assert response =~ "must have the @ sign and no spaces"
+    end
+  end
+end