refactor(search): support local/mirrored FlexSearch assets (#956)

* feat(search): support local and mirrored FlexSearch assets

Add params.search.flexsearch.base/js overrides in search script loading.

Document offline/local and mirror-based script asset configuration examples.

* refactor(scripts): normalize remote asset URL joins

* docs(config): clarify local asset examples

* docs(i18n): add local asset config guidance

layouts/_partials/scripts/mermaid.html

@@ -28,7 +28,7 @@
 {{- /* JS retrieval: get raw JS from either local asset or remote, then process */ -}}
 {{- if $isRemoteBase -}}
   {{- $jsPath := cond (ne $mermaidJsAsset "") $mermaidJsAsset (printf "mermaid%s.js" $minSuffix) -}}
-  {{- $mermaidJsUrl := printf "%s/%s" $mermaidBase $jsPath -}}
+  {{- $mermaidJsUrl := urls.JoinPath $mermaidBase $jsPath -}}
   {{- with try (resources.GetRemote $mermaidJsUrl) -}}
     {{- with .Err -}}
       {{- errorf "Could not retrieve Mermaid js file from %s. Reason: %s." $mermaidJsUrl . -}}