Add zero-pad option for numbered chapter titles

A "Zero-pad #" checkbox pads chapter titles that default to their
position number with leading zeros to a uniform width (the digit count
of the largest chapter number, e.g. 03. of 12). Padding never applies to
"# Title:" headers or manual overrides. The setting persists in config.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/config.rs

@@ -16,6 +16,10 @@ pub struct Config {
     /// An empty value disables header splitting.
     #[serde(default = "default_marker")]
     pub draft_marker: String,
+    /// When a chapter title defaults to its position number, pad it with leading
+    /// zeros to the width of the largest chapter number (e.g. "03." of 12).
+    #[serde(default)]
+    pub zero_pad_index: bool,
 }
 
 /// Default header/body separator, matching the manuscript drafting convention.
@@ -32,6 +36,7 @@ impl Default for Config {
             export_path,
             show_preview: false,
             draft_marker: default_marker(),
+            zero_pad_index: false,
         }
     }
 }