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/mod.rs

@@ -23,10 +23,11 @@ mod diff;
 mod editor;
 mod file_list;
 mod find;
+mod grammar;
 mod outline;
 mod project;
-mod grammar;
 mod spelling;
+mod story_bible_editor;
 mod story_bible_ui;
 mod style;
 mod ui;
@@ -381,6 +382,12 @@ pub struct App {
     selected_bible_item: Option<String>,
     /// References found in the current draft file.
     current_references: Vec<crate::story_bible::StoryBibleReference>,
+    /// Whether the Story Bible breakout editor window is open.
+    show_story_bible_editor: bool,
+    /// The content being edited in the Story Bible editor.
+    story_bible_editor_content: String,
+    /// The workspace-relative path of the file being edited in the Story Bible editor.
+    story_bible_editor_path: Option<String>,
 }
 
 impl App {
@@ -485,6 +492,9 @@ impl App {
             show_story_bible: false,
             selected_bible_item: None,
             current_references: Vec::new(),
+            show_story_bible_editor: false,
+            story_bible_editor_content: String::new(),
+            story_bible_editor_path: None,
         };
         app.load_spell_dict();
         app.open_workspace();
@@ -611,6 +621,10 @@ impl eframe::App for App {
             self.story_bible_window(ctx);
         }
 
+        if self.show_story_bible_editor {
+            self.story_bible_editor_window(ctx);
+        }
+
         if self.show_manuscript_settings {
             self.manuscript_settings_window(ctx);
         }