feat(clipboard): paste images from other programs on desktop too

A paste that finds no text now reads an image off the system clipboard
on desktop as well as on the web. The pixels are written as a PNG beside
the drawing (or in the config folder for a drawing never saved) and
placed as a raster image at the cursor, the same way the web paste
attaches a screenshot. PASTESPEC falls back to the same image paste when
the clipboard holds neither objects nor text.

Refs #645

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/mod.rs

@@ -2389,11 +2389,9 @@ pub enum Message {
     PasteSpecial(Option<String>),
     /// Completion of a system text clipboard read requested by PASTECLIP.
     SystemClipboardPaste(SystemClipboardText),
-    /// Web: completion of the clipboard-image read the paste fallback starts
-    /// when no text was found — `Some((name, bytes))` attaches the image,
-    /// `None` reports the usual unsupported-content error. Native never
-    /// constructs it.
-    #[cfg(target_arch = "wasm32")]
+    /// Completion of the clipboard-image read the paste fallback starts when
+    /// no text was found — `Some((name, bytes))` attaches the image, `None`
+    /// reports the usual unsupported-content error.
     SystemClipboardPasteImage(Option<(String, Vec<u8>)>),
     /// Ctrl/Cmd+A — select all layer rows when the Layer Manager is open, or all
     /// drawing objects otherwise (#236).