feat: Add Story Bible breakout editor window

- Add new module src/app/story_bible_editor.rs for dedicated Story Bible editing
- Add 'Edit -> Story Bible' menu option to open the editor
- Tree view (left 1/3) shows Story Bible directory structure with collapsible folders
- Editor (right 2/3) allows editing selected Story Bible items
- Auto-load first file on open, support Ctrl+S to save
- Rebuild Story Bible index after saving to pick up changes
- Update README.md with Story Bible Editor documentation

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

src/app/ui.rs

@@ -321,6 +321,10 @@ impl App {
                         ui.close_menu();
                         self.open_find(true);
                     }
+                    if ui.button("📖 Story Bible…").clicked() {
+                        ui.close_menu();
+                        self.open_story_bible_editor();
+                    }
                 });
                 ui.menu_button("Tools", |ui| {
                     if ui.button("☑ Outline…").clicked() {