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>

config/config.exs

@@ -57,17 +57,10 @@ config :esbuild,
   simpleshop_theme_shop_css: [
     args: ~w(css/shop.css --bundle --outdir=../priv/static/assets/css),
     cd: Path.expand("../assets", __DIR__)
-  ]
-
-# Configure tailwind (the version is required)
-config :tailwind,
-  version: "4.1.7",
-  simpleshop_theme: [
-    args: ~w(
-      --input=assets/css/app.css
-      --output=priv/static/assets/css/app.css
-    ),
-    cd: Path.expand("..", __DIR__)
+  ],
+  simpleshop_theme_admin_css: [
+    args: ~w(css/admin.css --bundle --outdir=../priv/static/assets/css),
+    cd: Path.expand("../assets", __DIR__)
   ]
 
 # Configures Elixir's Logger