Read PNG clipboard images, stub other platforms

[treeform]

Summary

• macOS getClipboardImage / getClipboardContentKinds now accept PNG as well as TIFF (screenshots and many apps provide both).
• Add no-op getClipboardImage / getClipboardContentKinds stubs on Linux (X11) and Emscripten so the API is available on every backend (Windows already had full support).

Test plan

• On macOS: copy a screenshot to the clipboard (Cmd+Ctrl+Shift+4), run a small snippet that calls getClipboardContentKinds() and getClipboardImage(), confirm non-nil image with expected size
• On macOS: copy plain text only, confirm ImageContent is absent and getClipboardImage() returns nil
• Linux/Emscripten still compile with the new stubs

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