Fix Story Bible Editor's content panel rendering a sliver wide

ui.set_width() was called on the shared horizontal-layout Ui before the
tree and editor panels were split off, so it shrank the whole row —
tree, separator, and editor panel together — down to a third of the
window's width instead of just the tree column. The editor's own
heading (the open item's path) was narrow enough to wrap one letter
per line as a result. Moving the width constraint onto the tree
panel's own child Ui, where it belongs, leaves the editor panel free
to take the rest of the row.

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

src/app/story_bible_editor.rs

@@ -231,11 +231,16 @@ impl App {
         // that borrow `self` have finished.
         let mut focused = false;
         ui.horizontal(|ui| {
-            // Left panel: Tree view (1/3 width)
+            // Left panel: Tree view (1/3 width). The width must be set on
+            // this panel's own child `Ui`, not the shared horizontal one —
+            // that had been shrinking the whole row (tree, separator, and
+            // the editor panel with it) down to a third of the window,
+            // which is why the editor's heading used to wrap one letter at
+            // a time.
             let tree_width = ui.available_width() / 3.0;
-            ui.set_width(tree_width);
-            
+
             ui.vertical(|ui| {
+                ui.set_width(tree_width);
                 ui.heading("Story Bible");
                 ui.separator();