feat: add dark mode support for generated flow diagrams

[xiapeli]

Summary

FlowMaster generates Mermaid diagrams with default (light) styling. Many developers use dark-themed editors and documentation sites. The generated diagrams should support a dark mode color scheme.

What needs to happen

1. Define a dark mode color palette for diagram elements:
   • Node fill colors (dark backgrounds with light text)
   • Edge/arrow colors (visible on dark backgrounds)
   • Subgraph background colors
2. Add a theme toggle instruction in the skill file (e.g., a parameter or flag for dark/light)
3. Use Mermaid's built-in theming system where possible:
   • `%%{init: {"theme": "dark"}}%%` directive
   • Custom `themeVariables` for FlowMaster-specific styling
4. Ensure all node types remain distinguishable in dark mode

Acceptance criteria

• Diagrams render correctly on dark backgrounds (GitHub dark mode, VS Code dark themes)
• All node type colors have sufficient contrast in dark mode
• Light mode remains the default (no regression)
• User can request dark mode via a clear instruction

Getting started

1. Review Mermaid theming docs: https://mermaid.js.org/config/theming.html
2. Test current FlowMaster output with `%%{init: {"theme": "dark"}}%%`
3. Identify which node styles need color overrides