feat(mermaid-editor): implement PNG export with direct file save and e2e tests by GCWing · Pull Request #140

feat(mermaid-editor): implement PNG export with direct file save and e2e tests by GCWing · Pull Request #140 · GCWing/BitFun

… e2e tests

- Refactor export to save PNG directly to downloads folder via Tauri fs API
- Simplify export button from SVG/PNG dropdown to a single PNG export button
- Expose getSvgElement/getSvgDimensions via MermaidPreview ref for export use
- Rewrite SVG-to-PNG conversion using Image/Canvas instead of blob URL approach
- Integrate notification system to show loading/success/error during export
- Prepare self-contained export SVG: strip interactive styles, add background rect
- Add data-testid attributes to NotificationButton, NotificationCenter, MermaidEditor
- Add l1-mermaid-export e2e test spec and corresponding npm script
- Add .bitfun/ to e2e .gitignore
- Add export i18n keys for en-US and zh-CN

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