fix: use e.button === 1 for pointerup handlers instead of isMiddlePointerInput

On pointerup, e.buttons reflects post-release state. Releasing LEFT while
MIDDLE is held gives e.buttons=4, causing isMiddlePointerInput to
false-positive. e.button is authoritative on up events, so use it directly
in all three pointerup sites: LGraphCanvas.processMouseUp, useStringWidget,
useMarkdownWidget.

src/lib/litegraph/src/LGraphCanvas.ts

@@ -3849,7 +3849,7 @@ export class LGraphCanvas implements CustomEventDispatcher<LGraphCanvasEventMap>
           this
         )
       }
-    } else if (isMiddlePointerInput(e)) {
+    } else if (e.button === 1) {
       this.dirty_canvas = true
       this.dragging_canvas = false
     } else if (e.button === 2) {