[jtsang4]

Add support for rendering Mermaid flowcharts, sequence diagrams, and other diagram types in agent replies, replacing the code block with the rendered SVG.

Changes

• Mermaid integration: Display ```mermaid code blocks as diagrams, not code
• Design system theming: Diagrams use theme colors (surface-2, input-strong, foreground, etc.) and match the design system's radius and styling
• Copy source: The 复制代码 button still copies the diagram source, not the SVG
• Wide diagram scrolling: Diagrams wider than 85% of viewport width shrink to 85%, then scroll sideways (same as code blocks)
• Lazy loading: Mermaid loads only when the first diagram appears, not with the app startup
• Graceful fallback: If Mermaid cannot parse the source, the block stays as code
• Security: Mermaid runs at strict security level, output is sanitized with DOMPurify

Testing

• Added E2E test verifying diagrams are drawn, colored correctly, and copy works
• Added unit tests for parsing, sanitization, and fallback behavior
• Updated screenshot and acceptance test setup to include a Mermaid diagram example
• Fakeagent now responds to "流程图" with a flowchart example