Strip comments and header metadata on ODT export

Each file is now cleaned before conversion: HTML comments are removed
and everything above the configurable Draft marker (default
"### Rough Draft:") is dropped. A "# Title:" line becomes the chapter
heading and a "# Slug:" line becomes an italic caption beneath it.

- new preprocess module (strip_comments + header parse) with tests
- Chapter gains a slug rendered via a Chapter_20_Caption style, italic
  at both paragraph-style and character-run level
- draft_marker config setting + top-bar field
- export title priority: override > # Title: > first # heading > filename

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

src/app.rs

@@ -276,21 +276,33 @@ impl App {
 
     fn export_odt(&mut self) {
         self.save_current();
+        let marker = self.config.draft_marker.clone();
         let mut chapters = Vec::new();
         for name in &self.files {
-            let markdown = std::fs::read_to_string(self.path_for(name)).unwrap_or_default();
-            // The leading `# heading` is always consumed as the chapter-title
-            // slot; a manual override, if set, replaces the derived title.
-            let (auto_title, body) = split_title(&markdown, name);
+            let raw = std::fs::read_to_string(self.path_for(name)).unwrap_or_default();
+            // Strip comments + the editorial header, and lift out any
+            // `# Title:` / `# Slug:` metadata.
+            let header = crate::preprocess::parse(&raw, &marker);
+
+            // Title priority: manual override > `# Title:` metadata > the body's
+            // leading `# heading` (consumed) > file name. When a title comes from
+            // metadata/override, the body is left intact; otherwise the leading
+            // `# heading` is consumed into the title slot to avoid duplication.
+            let (title, body) = if let Some(t) = &header.title {
+                (t.clone(), header.body.clone())
+            } else {
+                split_title(&header.body, name)
+            };
             let title = self
                 .titles
                 .get(name)
                 .map(|t| t.trim())
                 .filter(|t| !t.is_empty())
                 .map(|t| t.to_string())
-                .unwrap_or(auto_title);
+                .unwrap_or(title);
             chapters.push(Chapter {
                 title,
+                slug: header.slug.clone(),
                 markdown: body,
             });
         }
@@ -376,6 +388,20 @@ impl App {
                 {
                     self.config.save();
                 }
+                ui.separator();
+                ui.label("Draft marker:")
+                    .on_hover_text(
+                        "Text above this line in each file (its header) is dropped on export; \
+                         # Title: / # Slug: lines become the chapter heading and caption. \
+                         Leave blank to export files whole.",
+                    );
+                let marker_resp = ui.add(
+                    egui::TextEdit::singleline(&mut self.config.draft_marker)
+                        .desired_width(140.0),
+                );
+                if marker_resp.lost_focus() {
+                    self.config.save();
+                }
             });
             ui.add_space(4.0);
         });