switch to Roboto web font (#22)

for now, at least. tired of waiting on Chromium to fix system fonts on Catalina.... https://bugs.chromium.org/p/chromium/issues/detail?id=1005969

Roboto seems closest to San Francisco.

layouts/index.headers

@@ -1,17 +1,18 @@
 # Sensible default security headers
 #  - More info: https://scotthelme.co.uk/hardening-your-http-response-headers/
 #  - Test website: https://securityheaders.com/
-
 /*
   Referrer-Policy: strict-origin-when-cross-origin
   X-Content-Type-Options: nosniff
   X-Frame-Options: sameorigin
   X-XSS-Protection: 1; mode=block
 
+# Super long cache for web fonts (one year)
+/fonts/*
+  Cache-Control: max-age=31536000, public, immutable, no-transform
 
 # Recommended MIME type for PWA manifests
 # https://github.com/w3c/manifest/issues/689
-
 /manifest.json
   Content-Type: application/manifest+json; charset=UTF-8
 /*.webmanifest