The bottom format toolbar (Raw and Split raw pane) includes a Mermaid… combo box next to the table-of-contents control. Each entry dispatches MarkdownFormatCommand::InsertMermaid(MermaidTemplateKind) through the same deferred path as other format actions (RibbonAction::Format in src/app/central_panel.rs).
Starter diagram bodies live in one module: src/markdown/mermaid/templates.rs (MermaidTemplateKind and snippet_body()). They are wrapped as fenced mermaid code blocks in apply_mermaid_snippet_format in src/markdown/formatting.rs (adds surrounding newlines when inserting mid-line).
Labels use format_toolbar.mermaid_* keys in locales/en.yaml.