perf: split CSS bundles for shop and admin pages

Create separate CSS bundles to reduce shop page load times:
- app-shop.css (45KB/7.8KB gzip): Shop pages only, no daisyUI
- app.css (139KB): Admin pages with daisyUI and theme editor

Key changes:
- Add app-shop.css with targeted @source paths for shop files only
- Move .preview-frame rules from theme-layer2-attributes.css to app.css
- Delete fonts.css (fonts now generated inline by CSSGenerator)
- Add inline all-fonts generation in theme editor for typography switching
- Configure separate Tailwind profiles and watchers for both bundles

Shop pages now load 54% less CSS by excluding:
- daisyUI components (admin only)
- .preview-frame theme switching rules (editor only)
- Admin-specific Tailwind utilities

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

mix.exs

@@ -86,9 +86,15 @@ defmodule SimpleshopTheme.MixProject do
       "ecto.reset": ["ecto.drop", "ecto.setup"],
       test: ["ecto.create --quiet", "ecto.migrate --quiet", "test"],
       "assets.setup": ["tailwind.install --if-missing", "esbuild.install --if-missing"],
-      "assets.build": ["compile", "tailwind simpleshop_theme", "esbuild simpleshop_theme"],
+      "assets.build": [
+        "compile",
+        "tailwind simpleshop_theme",
+        "tailwind simpleshop_theme_shop",
+        "esbuild simpleshop_theme"
+      ],
       "assets.deploy": [
         "tailwind simpleshop_theme --minify",
+        "tailwind simpleshop_theme_shop --minify",
         "esbuild simpleshop_theme --minify",
         "phx.digest"
       ],