feat: Implement bidirectional Story Bible links with draft text

- Add story_bible.rs module with StoryBibleIndex, StoryBibleItem, and
  StoryBibleReference types for indexing Story Bible content
- Support wiki-style [[...]] and mention-style @... syntax for references
- Highlight Story Bible references in editor with purple background
- Add Story Bible window (View > Tools > Story Bible) to browse items
- Show references from current draft file in Story Bible window
- Allow clicking references to jump to their location in the editor
- Auto-index Story Bible items from common directories (03-Characters,
  02-Story Bible, Characters, etc.)
- Extract metadata from YAML frontmatter or filenames
- Add walkdir dependency for directory scanning
- Fix pre-existing format string errors in workspace.rs

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

src/app/mod.rs

@@ -27,6 +27,7 @@ mod outline;
 mod project;
 mod grammar;
 mod spelling;
+mod story_bible_ui;
 mod style;
 mod ui;
 mod util;
@@ -372,6 +373,14 @@ pub struct App {
     watcher_rx: Option<mpsc::Receiver<WatcherMsg>>,
     /// The file watcher thread handle, so it can be stopped when the workspace closes.
     watcher_thread: Option<thread::JoinHandle<()>>,
+    /// Index of Story Bible items for cross-referencing with draft text.
+    story_bible_index: crate::story_bible::StoryBibleIndex,
+    /// Whether the Story Bible window is open.
+    show_story_bible: bool,
+    /// Currently selected Story Bible item path.
+    selected_bible_item: Option<String>,
+    /// References found in the current draft file.
+    current_references: Vec<crate::story_bible::StoryBibleReference>,
 }
 
 impl App {
@@ -472,6 +481,10 @@ impl App {
             show_project_settings: false,
             watcher_rx: None,
             watcher_thread: None,
+            story_bible_index: crate::story_bible::StoryBibleIndex::new(),
+            show_story_bible: false,
+            selected_bible_item: None,
+            current_references: Vec::new(),
         };
         app.load_spell_dict();
         app.open_workspace();
@@ -594,6 +607,10 @@ impl eframe::App for App {
             self.characters_window(ctx);
         }
 
+        if self.show_story_bible {
+            self.story_bible_window(ctx);
+        }
+
         if self.show_manuscript_settings {
             self.manuscript_settings_window(ctx);
         }