feat: support saving original image

[eszlamczyk]

What/Why?

Closes #380

Long-pressing an image in EnrichedMarkdownText and tapping Save to Photos saved the processed render instead of the source image. Instrumentation of ENRMImageAttachment showed that UIKit's attachment context menu, save action, and drag session all read the plain NSTextAttachment.image property (_attachmentHasImage:, _imageFromAttachment:, _defaultMenuForInteractableItem), which we populated with:

• inline images: the scaled, border-radius-clipped bitmap - saved image was tiny and clipped,
• block images re-created via the processed-image cache: a stale 1×1 placeholder - saved image was a blank pixel.

The fix separates the two roles the image property was playing:

• image now always holds the original downloaded image - set as soon as the download completes, on both the fresh-process and cache-hit paths. This is what UIKit serves to Save to Photos, Copy Image, drag-and-drop, and RTFD copy/paste.
• The processed (scaled + border-radius) bitmap lives only in loadedImage, returned from imageForBounds: for display - so on-screen rendering is unchanged.
• The 1×1 loading placeholder moved to its own placeholderImage property. Before an image finishes loading, image is nil, so the system no longer offers image actions that would save a blank pixel.

Testing

In the example app (iOS simulator), with logging on the attachment accessors:

1. Long-press a block image - Save to Photos => saved image is the full-resolution original (800×207 instead of the clipped 341×200 render).
2. Long-press an inline image → Save to Photos → original 80×73 instead of the processed 20×20 thumbnail.
3. Re-add an image so the attachment is rebuilt from the processed-image cache → long-press save still gets the original (previously a blank 1×1 placeholder).
4. Verified display is unchanged: imageForBounds: still returns the processed bitmap for drawing (sizing and border radius intact), and the drag preview still shows the rendered view.

Screenshots

Note the cut corners

	Before	After
Image after saving

PR Checklist

• Code compiles and runs on iOS
• Code compiles and runs on Android
• Updated documentation/README if applicable
• Ran example app to verify changes
• E2E tests are passing
• Required E2E tests have been added (if applicable)

[Copilot]

Pull request overview

Updates the iOS ENRMImageAttachment behavior so UIKit “image actions” (Save to Photos / Copy Image / drag-and-drop) use the original downloaded image rather than the processed (scaled/clipped) render, while keeping on-screen rendering unchanged.

Changes:

• Separates “original image for system actions” (NSTextAttachment.image) from “processed image for display” (loadedImage via imageForBounds:).
• Introduces a dedicated placeholderImage so image can remain nil until the real image is available.

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