Fix #2091 copy/paste masks between layers

Edit > Copy with masks selected copied the layer, so Paste duplicated it
instead of adding the mask. Masks now go to their own clipboard and Paste
appends them, with unique names, to the selected unlocked layers.

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

crates/engine/src/cmd/effect.rs

@@ -851,6 +851,7 @@ fn copy(s: &mut Session, p: &Value) -> Result<Value> {
     s.state.key_clipboard.clear();
     s.state.keyframe_text = None;
     s.state.contents_clipboard.clear();
+    s.state.mask_clipboard.clear();
     s.state.link_clipboard = None;
     s.state.clip_is_keys = false;
     let n = groups.len();