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>

docs/plans/admin-font-loading.md

@@ -1,6 +1,6 @@
 # Admin font loading
 
-Status: Open — needs discussion
+Status: Complete (Option A + cache miss fix)
 
 ## Problem