feat: add sharing of the current canvas as a PNG image.

[jShubh-AD]

Adds a Share Image option to the overflow menu, allowing users to share the current canvas as a PNG image using share_plus. Includes integration tests verifying the share action.

New Features and Enhancements

• Added Share Image option to the overflow menu in the workspace.
• Used share_plus to trigger the platform share sheet.
• The shared image is generated using the existing canvas export pipeline (RenderImageForExport), ensuring the shared image matches the saved image output.

Refactorings and Bug Fixes

• No refactoring or bug fixes included in this PR.

Checklist

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