Update dockview-core 4.13.1→5.0.0 and mermaid 11.12.2→11.12.3

Replace bundled JS/CSS assets with latest versions. dockview-core 5.0.0
is a major bump but all APIs used by ICL are unchanged. mermaid 11.12.3
is a patch release.

Also fix a reader-time SLYNK package error in image-cache.lisp by
replacing bare slynk:create-server with a runtime find-symbol lookup.

Closes #22.

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