feat: scope the sidebar to the current category

Assisted-by: Delta:zai-glm-5.3

site/calepin.toml

@@ -1,7 +1,7 @@
 title = "Yggdrasil"
 description = "Personal knowledge base."
 base-url = "https://yggdrasil.kb28.ch"
-theme = "calepin"
+theme = "themes/yggdrasil"
 pdf = false
 minify = true
 search = "pagefind"