ci: enforce strict registry auth and optimize quarto builds

Replace the conditional Docker login check with a direct, non-interactive authentication call that fails immediately on invalid credentials. Update comments to clarify skopeo versus Docker credential store behavior. Additionally, add `freeze: auto` to the Quarto configuration to automatically cache code cell outputs, preventing redundant re-computation and optimizing build performance.

web_src/_quarto.yml

@@ -4,6 +4,9 @@ project:
   post-render:
     - scripts/copy-to-web.sh
 
+execute:
+  freeze: auto
+
 format:
   html:
     toc: false