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/lib/firehose_web/controllers/user_session_html.ex

@@ -0,0 +1,9 @@
+defmodule FirehoseWeb.UserSessionHTML do
+  use FirehoseWeb, :html
+
+  embed_templates "user_session_html/*"
+
+  defp local_mail_adapter? do
+    Application.get_env(:firehose, Firehose.Mailer)[:adapter] == Swoosh.Adapters.Local
+  end
+end