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>

README.md

@@ -69,7 +69,9 @@ Terminal=false
    auto-derived one (the field shows the automatic title as a hint). Leaving it
    blank falls back to a `# Title:` line in the header, and if there is none, to
    the chapter's **position in the left pane** followed by a period (`1.`, `2.`,
-   …). Titles are saved to `titles.json` and sync with git.
+   …). Tick **Zero-pad #** in the top bar to pad those numbered titles with
+   leading zeros to a uniform width (e.g. `03.` when there are 12 chapters).
+   Titles are saved to `titles.json` and sync with git.
 5. Set the **Export** path (type it, or click **📂** to choose the `.odt` file in a
    native save dialog) and press **Export ODT** to produce the concatenated
    document. Each file becomes a chapter headed by its title, optionally followed