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>

README.md

@@ -391,6 +391,21 @@ markdown file with YAML frontmatter containing `id:` and `title:` fields will be
 included in the index. References in your draft text that match Story Bible items
 are highlighted and clickable.
 
+### Story Bible Editor (Edit ▸ Story Bible…)
+
+For a dedicated editing experience, use the **Story Bible breakout editor**. This
+provides a focused workspace for managing your Story Bible items:
+
+* **Tree view** (left 1/3 of window): Shows the complete Story Bible directory structure
+  with collapsible folders for Characters, Locations, Lore, and other categories
+* **Editor** (right 2/3 of window): Full-featured text editor for the selected item
+* **Auto-load**: Select any file from the tree to load it into the editor
+* **Save**: Click **Save** or press `Ctrl+S` to write changes back to disk
+* **Index updates**: Saving a file automatically rebuilds the Story Bible index
+
+The editor is ideal for extended Story Bible work, allowing you to browse and edit
+items without leaving the context of your reference material.
+
 ### Outline (Tools ▸ Outline…)
 
 The snowflake outline files are scaffolds of HTML comments, one per beat