fix(shell): select a newly added library folder in the tree

Adding a root left it in the tree but unselected -- the user had to
go find and click it themselves. addRoot now selects it right away
(onFolderSelected then jumps to Photos and filters to it, same as a
manual click), and replaceRoot carries that selection over to the
rebuilt row once the folder's scan finishes, the same way it already
carries over the expanded state.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/main/java/org/icroco/pholio/ui/shell/LibraryFolderTree.java

@@ -215,6 +215,10 @@ public class LibraryFolderTree extends StackPane implements Disposable {
         tree.getRoot().getChildren().add(item);
         tree.getRoot().getChildren().sort(BY_DISPLAY_NAME);
         updateEmptyState();
+        // A root the user just picked should read as "here it is, already showing" rather than leaving
+        // them to go find and click it themselves — onFolderSelected then does the rest (jumps to Photos,
+        // filters the gallery to it) exactly as a manual click would.
+        tree.getSelectionModel().select(item);
     }
 
     /**
@@ -245,8 +249,11 @@ public class LibraryFolderTree extends StackPane implements Disposable {
     }
 
     /**
-     * Swaps in a freshly built root item, carrying over whether the stale one was expanded. A no-op if the
-     * root is gone by the time the scan finishes — added and removed again before its walk completed.
+     * Swaps in a freshly built root item, carrying over whether the stale one was expanded — and, since a
+     * just-added root ({@link #addRoot}) is selected before its scan ever finishes, whether it was selected
+     * too, or that selection would otherwise vanish under it the moment the scan's own rebuild lands. A
+     * no-op if the root is gone by the time the scan finishes — added and removed again before its walk
+     * completed.
      */
     @FxThread
     private void replaceRoot(Path path, FolderTreeItem refreshed) {
@@ -254,7 +261,11 @@ public class LibraryFolderTree extends StackPane implements Disposable {
         for (int i = 0; i < roots.size(); i++) {
             if (roots.get(i).getValue().equals(path)) {
                 refreshed.setExpanded(roots.get(i).isExpanded());
+                boolean wasSelected = roots.get(i).equals(tree.getSelectionModel().getSelectedItem());
                 roots.set(i, refreshed);
+                if (wasSelected) {
+                    tree.getSelectionModel().select(refreshed);
+                }
                 return;
             }
         }