feat: use Phoenix config for theme injection (cleaner approach)

asciinema/custom.exs

@@ -15,3 +15,15 @@ config :asciinema, Asciinema.Emails.Mailer,
     verify: :verify_none,
     versions: [:"tlsv1.2", :"tlsv1.3"]
   ]
+
+# Serve custom theme files as static assets
+config :asciinema, AsciinemaWeb.Endpoint,
+  static_url: [path: "/theme"],
+  check_origin: false
+
+# Try to inject custom head HTML (if supported by asciinema)
+config :asciinema, AsciinemaWeb.Endpoint,
+  extra_head_html: """
+  <link rel="stylesheet" href="/theme/custom.css">
+  <link rel="icon" type="image/svg+xml" href="/theme/favicon.svg">
+  """