Page-break between chapters and numeric title fallback

Every chapter after the first now starts on a new page via a
Chapter_20_Break heading style (fo:break-before="page"); the first
chapter keeps the plain heading so there is no blank leading page.

Chapter titles now resolve as: manual override > "# Title:" header >
the chapter's 1-based position in the left pane (e.g. "3."). The old
"# heading" / filename fallbacks are replaced by this numbering, and the
same logic drives the chapter-title hint.

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

README.md

@@ -67,13 +67,14 @@ Terminal=false
    to write to disk. Drag the `⠿` handles to reorder.
 4. Set the **Chapter title** for the open file if you want to override the
    auto-derived one (the field shows the automatic title as a hint). Leaving it
-   blank falls back to a `# Title:` line, then the file's first level-1 heading
-   (`# ...`), then the file name. Titles are saved to `titles.json` and sync with
-   git.
+   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.
 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
-   by a caption from its `# Slug:` line.
+   by a caption from its `# Slug:` line. **Every chapter starts on a new page**
+   (the first chapter excepted, so there is no blank leading page).
 
 The status bar shows a live **word count** for the current file, plus the net
 words added (or removed) to it since the current session started. The count