feat: Add auto-snapshot feature with git commit every N words

- Add new module src/app/auto_snapshot.rs for auto-snapshot functionality
- Add configuration options: auto_snapshot_enabled (default: false) and
  auto_snapshot_threshold (default: 250 words)
- Automatically create git commits when word count crosses threshold boundaries
- Add 'Edit -> Snapshots' menu option to open snapshots window
- Snapshots window shows: enable/disable toggle, threshold setting, current
  word count, next snapshot target, list of all snapshots with rollback buttons
- Manual snapshot creation via button in snapshots window
- Rollback to any previous snapshot with one click
- Snapshot info includes: word count, timestamp, commit message, commit hash
- Make run_git in gitsync.rs public for use by auto_snapshot module
- Update README.md with Auto-Snapshots documentation

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

src/app/ui.rs

@@ -325,6 +325,10 @@ impl App {
                         ui.close_menu();
                         self.open_story_bible_editor();
                     }
+                    if ui.button("📸 Snapshots…").clicked() {
+                        ui.close_menu();
+                        self.show_snapshots = true;
+                    }
                 });
                 ui.menu_button("Tools", |ui| {
                     if ui.button("☑ Outline…").clicked() {
@@ -581,6 +585,9 @@ impl App {
                     } else {
                         self.editor(ui);
                     }
+                    
+                    // Check for auto-snapshot after editor updates
+                    self.check_auto_snapshot();
                 }
                 None => {
                     ui.centered_and_justified(|ui| {