dsh-mermaid

Renders ```mermaid code fences in DSH Web chat messages as SVG diagrams, with viewport-driven and async-queue rendering so long conversations stay responsive. The chart theme follows the GUI light/dark mode, and a fullscreen overlay adds wheel zoom and drag-to-pan. securityLevel is locked to strict with sanitized labels and no bound click handlers; on render failure the source block is kept with a copyable error summary that can be sent to the AI for fixing.