feat(mermaid-editor): implement PNG export with direct file save and e2e tests

[GCWing]

Summary

• Refactored Mermaid editor PNG export to save directly to the user's downloads folder via Tauri fs API, replacing the old onExport callback pattern
• Simplified the export UI from an SVG/PNG dropdown to a single PNG export button with data-testid for testability
• Rewrote SVG-to-PNG conversion pipeline: prepare a self-contained SVG clone (strip interactive styles, add background rect, fix namespaces), load via Image, draw onto Canvas, export as Blob
• Integrated notification system to show loading/success/error feedback during export
• Exposed getSvgElement and getSvgDimensions on MermaidPreview ref for use by the export handler
• Added data-testid attributes to NotificationButton, NotificationCenter, and MermaidEditor for E2E testability
• Added new E2E test spec l1-mermaid-export.spec.ts and corresponding test:l1:mermaid-export npm script
• Added .bitfun/ snapshot directory to tests/e2e/.gitignore
• Added export-related i18n keys for en-US and zh-CN

Test plan

• Open Mermaid editor, click the export (Download) button, verify PNG is saved to downloads folder
• Verify notification shows loading state during export, then success with file path
• Verify error notification is shown if export fails
• Run npm run test:l1:mermaid-export in tests/e2e to validate E2E test passes