feat: Add PNG Image Export Support

[Sahaj33-op]

This PR adds the ability to export the markdown preview as a PNG image.

Changes

• Added Image (.png) to the Export dropdown in the desktop and mobile toolbars.
• Implemented exportPng utilizing html2canvas and FileSaver to capture the DOMPurify sanitized preview, identical to the PDF capture approach.
• Propagated translation mappings for the new dropdown option.
• Synchronized desktop-app/resources for Neutralinojs compatibility.

[Sahaj33-op]

Output image of default markdown text:

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Project	Deployment	Actions	Updated (UTC)
markdown-viwer	Ready	Preview, Comment	Jun 16, 2026 4:54am

[ThisIs-Developer]

@Sahaj33-op Thank you for the PR! It's a great idea. I've reviewed the PR and merged it into the main branch. Thanks for your contribution!

[ThisIs-Developer]

@Sahaj33-op Thank you again for adding PNG export support!

After merging the feature, I noticed a small issue during testing. The PNG export flow was still reusing parts of the PDF export modal logic, so when exporting an image the progress modal continued to display PDF-related labels such as "Generating PDF". The UI was also targeting the PDF export triggers instead of the PNG export triggers.

I've addressed this in a follow-up PR #180 by making the progress modal and trigger handling aware of the export type, so PNG exports now display the correct labels and use the appropriate buttons.