feat(annotate): open .mmd and .dot files in the diagram viewer; core 0.25.5, ui 0.42.0 (#1571)

plannotator annotate flow.mmd / .mermaid / graph.dot / .gv now opens the file as one diagram in the existing DiagramBlock → DiagramViewer → DiagramPopout engine instead of being refused: themed, zoom/pan/popout, click-to-comment on nodes, edges and clusters, drafts, export with real file lines, version history and share links. Both the Bun server and the Pi mirror send renderAs: "mermaid" | "graphviz" on /api/plan and /api/doc. Adds diagramRenderKindForPath to @plannotator/core (0.25.5) and diagramDocumentBlocks, shareableDocumentMarkdown, DocumentRenderAs and Block.diagramSourceLineOffset to @plannotator/ui (0.42.0).