Fix bin editor handle hit-test mismatch

showModal() must be called before load_image() so the canvas has its
correct layout dimensions when parentElement.clientWidth is read.
Calling it after caused css_w to be computed against an unlaid-out
dialog, making drawn handle positions not match hit-test positions.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/app.mjs

@@ -2107,6 +2107,10 @@ function open_bin_editor(bin) {
 	bin_editor_bin_id = bin.id;
 	document.getElementById('bin-editor-name').value = bin.name;
 
+	// Show dialog first so the canvas has correct layout dimensions before
+	// load_image reads parentElement.clientWidth to size itself.
+	dlg.showModal();
+
 	const canvas = document.getElementById('bin-editor-canvas');
 	bin_editor_instance = new Grid_Setup(canvas);
 	bin_editor_instance.set_rows(1);
@@ -2116,8 +2120,6 @@ function open_bin_editor(bin) {
 			bin_editor_instance.set_corners(bin.corners);
 		}
 	});
-
-	dlg.showModal();
 }
 
 // ---------------------------------------------------------------------------