remove Tailwind + DaisyUI theme + heroicons plugin, admin fully custom CSS (Phase 7)

replace Tailwind CLI with esbuild for admin CSS bundling. admin now uses
hand-written utility classes (admin/utilities.css), static heroicon CSS
generated by mix generate_admin_icons, plain CSS colour themes extracted
from DaisyUI plugin config, and minimal resets. rename app.css to admin.css
for clarity alongside shop.css. delete vendor/daisyui-theme.js and
vendor/heroicons.js. no Tailwind dependency remains in the project.

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

lib/simpleshop_theme_web/controllers/error_html.ex

@@ -66,7 +66,7 @@ defmodule SimpleshopThemeWeb.ErrorHTML do
         <meta charset="utf-8" />
         <meta name="viewport" content="width=device-width, initial-scale=1" />
         <title>{@error_code} - {@error_title}</title>
-        <link phx-track-static rel="stylesheet" href={~p"/assets/css/app.css"} />
+        <link phx-track-static rel="stylesheet" href={~p"/assets/css/admin.css"} />
         <style id="theme-css">
           <%= Phoenix.HTML.raw(@generated_css) %>
         </style>