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/mod.rs

@@ -17,6 +17,7 @@ use std::thread;
 use std::time::{Duration, Instant};
 
 mod autocomplete;
+mod auto_snapshot;
 mod beats;
 mod characters;
 mod diff;
@@ -388,6 +389,12 @@ pub struct App {
     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>,
+    /// List of auto-snapshots for rollback.
+    snapshots: Vec<crate::app::auto_snapshot::SnapshotInfo>,
+    /// Word count at the last auto-snapshot.
+    last_snapshot_word_count: usize,
+    /// Whether the snapshots window is open.
+    show_snapshots: bool,
 }
 
 impl App {
@@ -495,6 +502,9 @@ impl App {
             show_story_bible_editor: false,
             story_bible_editor_content: String::new(),
             story_bible_editor_path: None,
+            snapshots: Vec::new(),
+            last_snapshot_word_count: 0,
+            show_snapshots: false,
         };
         app.load_spell_dict();
         app.open_workspace();
@@ -625,6 +635,10 @@ impl eframe::App for App {
             self.story_bible_editor_window(ctx);
         }
 
+        if self.show_snapshots {
+            self.snapshots_window(ctx);
+        }
+
         if self.show_manuscript_settings {
             self.manuscript_settings_window(ctx);
         }