Small refactor to move selection logic into tree-selection

Teach tree-selection how to determine if a node or any of its parents
are selected, and leverage that logic in the UI code (rather than
open-coding it in the UI directly).

src/model/tree-selection.ts

@@ -117,6 +117,17 @@ export class TreeSelection<
     for (const c of node.children) yield* this.selectedItemsInSubtree(c);
   }
 
+  /** Check if the provided node or any of its parents is selected.  Useful for
+   * precluding things like moving a node into a child of itself. */
+  isSelfOrParentSelected(node?: P | N): boolean {
+    while (node) {
+      const si = this.info(node);
+      if (si.isSelected) return true;
+      node = node.position?.parent;
+    }
+    return false;
+  }
+
   /** Set all `node.isSelected` properties to false within `this.roots`. */
   clearSelection(): void {
     this.lastSelected = undefined;

src/stash-list/folder.vue

@@ -443,13 +443,7 @@ export default defineComponent({
     },
 
     canMoveIntoFolder(): boolean {
-      let f: Folder | undefined = this.folder;
-      while (f) {
-        const si = the.model.selection.info(f);
-        if (si.isSelected) return false;
-        f = f.position?.parent;
-      }
-      return true;
+      return !the.model.selection.isSelfOrParentSelected(this.folder);
     },
   },