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/config.rs

@@ -11,6 +11,16 @@ pub struct Config {
     /// Whether to show the live preview pane.
     #[serde(default)]
     pub show_preview: bool,
+    /// Marker line separating a file's editorial header from its prose. Content
+    /// above it (except `# Title:` / `# Slug:` metadata) is dropped on export.
+    /// An empty value disables header splitting.
+    #[serde(default = "default_marker")]
+    pub draft_marker: String,
+}
+
+/// Default header/body separator, matching the manuscript drafting convention.
+pub fn default_marker() -> String {
+    "### Rough Draft:".to_string()
 }
 
 impl Default for Config {
@@ -21,6 +31,7 @@ impl Default for Config {
             workspace,
             export_path,
             show_preview: false,
+            draft_marker: default_marker(),
         }
     }
 }