Fix EpubMaker.MakeManifest to use relative path instead of full path

[Copilot]

The MakeManifest method expects a relative path (folder + filename), but was receiving a full absolute path after recent changes, breaking cover image matching in EPUB generation.

Changes

• Fixed MakeManifest call at line 555 to pass kImagesFolder + "/" + Path.GetFileName(epubThumbnailImagePath) instead of the full epubThumbnailImagePath
• Ensures cover image matching works correctly (relies on Path.GetFileNameWithoutExtension at line 749 and item comparison at line 823)

// Before: passed full path
MakeManifest(thumbnailFileExists ? epubThumbnailImagePath : null);

// After: passes relative path
MakeManifest(thumbnailFileExists ? kImagesFolder + "/" + Path.GetFileName(epubThumbnailImagePath) : null);

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