fix: use original file when sharing image with alpha channel

[BugenZhao]

No description provided.

[Copilot]

Pull request overview

This PR fixes image sharing behavior to preserve the original file format when sharing images with alpha channels. Previously, all eligible images were converted to JPEG, which would lose alpha channel information. The changes ensure that images with transparency are shared in their original format.

Changes:

• Added alpha channel detection to isPlainImage property to exclude images with alpha from JPEG conversion
• Added diagnostic logging when transferable image creation fails

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
app/Shared/Models/ViewingImageModel.swift	Adds alpha channel check to isPlainImage to prevent images with transparency from being converted to JPEG
app/iOS/Views/NewImageViewer.swift	Adds warning log when TransferableImage initialization fails for debugging purposes

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[Copilot]

The alpha channel detection logic may incorrectly identify images with .noneSkipFirst or .noneSkipLast alpha info as having an alpha channel. According to Apple's documentation, CGImageAlphaInfo has several cases that indicate no alpha channel:

• .none: No alpha channel
• .noneSkipLast: No alpha channel, ignore last component (e.g., RGB_555 format)
• .noneSkipFirst: No alpha channel, ignore first component (e.g., BGR_555 format)

The current check only compares against .none, which means images with .noneSkipFirst or .noneSkipLast would be incorrectly treated as having an alpha channel. This would cause them to be shared as the original file instead of being converted to JPEG, which is unnecessarily conservative.

Consider checking if the alpha info actually represents an alpha channel by using a more comprehensive check. For example:

let hasAlpha = cgImage?.alphaInfo.map { info in
  info != .none && info != .noneSkipFirst && info != .noneSkipLast
} ?? false
if hasAlpha { return false }

While this issue affects relatively rare image formats and fails conservatively (preserving quality), it's worth fixing for correctness.

Suggested change

	if cgImage?.alphaInfo ?? .none != .none { return false }
	let hasAlpha = cgImage?.alphaInfo.map { info in
	info != .none && info != .noneSkipFirst && info != .noneSkipLast
	} ?? false
	if hasAlpha { return false }