feat(superdeck)!: render Mermaid diagrams at runtime

[leoafarias]

Summary

• Render supported Mermaid fences directly in SuperDeck with flutter_mermaid,
  transparent backgrounds, light/dark theme support, and inline errors.
• Cover flowchart, sequence, pie, Gantt, timeline, Kanban, radar, and XY
  diagrams at compact and presentation resolutions.
• Remove superdeck_mermaid, Puppeteer, vendored JavaScript, generated PNGs,
  cache logic, obsolete CI coverage, and plugin setup documentation.

Breaking change

superdeck_mermaid, MermaidBuildPlugin, MermaidGenerator, browser
configuration, generated Mermaid assets, and the Mermaid PNG cache workflow are
removed in favor of runtime rendering without plugin registration or a custom
CLI runner.

Validation

• fvm dart run melos run build_runner:build --no-select
• fvm dart run melos run analyze:dart --no-select
• fvm dart run melos run test --no-select
• cd packages/superdeck && fvm dart analyze --fatal-infos
• cd packages/superdeck && fvm flutter test --exclude-tags ci-excluded
• macOS integration screenshot test for the runtime Mermaid slide

[github-actions]

Visit the preview URL for this PR (updated for commit e781d46):

https://superdeck-dev--pr104-feat-improve-mermaid-0iqesaz5.web.app

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