refactor: add themed form components for consistent shop styling

Adds reusable Phoenix components (shop_input, shop_textarea, shop_select,
shop_button, shop_card) backed by semantic CSS classes (.themed-input,
.themed-button, etc.) to eliminate repeated inline styles across templates.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/simpleshop_theme_web/live/user_live/login.ex

@@ -7,7 +7,7 @@ defmodule SimpleshopThemeWeb.UserLive.Login do
   def render(assigns) do
     ~H"""
     <Layouts.app flash={@flash} current_scope={@current_scope}>
-      <div class="mx-auto max-w-sm space-y-4">
+      <div class="mx-auto max-w-sm flex flex-col gap-4">
         <div class="text-center">
           <.header>
             <p>Log in</p>