fix admin font loading and theme CSS cache miss bug

add system font stack to admin reset CSS so setup/admin pages render
sans-serif on all devices instead of falling through to browser default.
pass path_resolver to CSSGenerator.generate on cache miss paths so
font URLs resolve to digested paths in production.

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

lib/berrypod_web/plugs/load_theme.ex

@@ -27,7 +27,7 @@ defmodule BerrypodWeb.Plugs.LoadTheme do
 
         :miss ->
           settings = Settings.get_theme_settings()
-          css = CSSGenerator.generate(settings)
+          css = CSSGenerator.generate(settings, &BerrypodWeb.Endpoint.static_path/1)
           CSSCache.put(css)
           {settings, css}
       end