replace orphaned tailwind classes with project CSS

auth pages (login, registration, confirmation, recover) now use
setup-page/setup-header/admin-btn-block. theme toggle indicator
gets proper CSS. cleaned up dead h-full, size-3.5, ml-2 classes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/berrypod_web/live/auth/registration.ex

@@ -8,15 +8,13 @@ defmodule BerrypodWeb.Auth.Registration do
   def render(assigns) do
     ~H"""
     <Layouts.app flash={@flash} current_scope={@current_scope}>
-      <div class="mx-auto max-w-sm">
-        <div class="text-center">
+      <div class="setup-page">
+        <div class="setup-header">
           <.header>
             Register for an account
             <:subtitle>
               Already registered?
-              <.link navigate={~p"/users/log-in"} class="font-semibold text-brand hover:underline">
-                Log in
-              </.link>
+              <.link navigate={~p"/users/log-in"} class="admin-link">Log in</.link>
               to your account now.
             </:subtitle>
           </.header>
@@ -32,7 +30,7 @@ defmodule BerrypodWeb.Auth.Registration do
             phx-mounted={JS.focus()}
           />
 
-          <.button phx-disable-with="Creating account..." variant="primary" class="w-full">
+          <.button phx-disable-with="Creating account..." variant="primary" class="admin-btn-block">
             Create an account
           </.button>
         </.form>