feat(core): support configurable list_margin in HTML list rendering

Add `HtmlRenderer::with_list_margin`. When the margin is >= 0 the
outermost list gets an inline `margin-left:<n>em`; nested lists and the
default of -1 stay unstyled (deferring to the stylesheet). Wires the
per-wiki vimwiki `list_margin` key into the export pipeline.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/nuwiki-core/src/render/html.rs

@@ -46,6 +46,10 @@ pub struct HtmlRenderer {
     /// names fall through to the default `class="color-<name>"`
     /// rendering. Matches vimwiki's `color_dic`.
     colors: HashMap<String, String>,
+    /// vimwiki `list_margin`: left margin (in `em`) applied to the
+    /// outermost list. `-1` (the default) emits no inline style and
+    /// defers to the stylesheet; `>= 0` forces `margin-left:<n>em`.
+    list_margin: i32,
 }
 
 impl Default for HtmlRenderer {
@@ -61,6 +65,7 @@ impl HtmlRenderer {
             template: None,
             vars: HashMap::new(),
             colors: HashMap::new(),
+            list_margin: -1,
         }
     }
 
@@ -95,6 +100,13 @@ impl HtmlRenderer {
         self
     }
 
+    /// Set the vimwiki `list_margin`. `-1` defers to the stylesheet;
+    /// `>= 0` forces a `margin-left:<n>em` on the outermost list.
+    pub fn with_list_margin(mut self, margin: i32) -> Self {
+        self.list_margin = margin;
+        self
+    }
+
     /// Supply a `color_dic`: vimwiki colour-tag names mapped to their
     /// CSS values. A name listed here is rendered as `style="color:V"`;
     /// missing names fall through to `class="color-<name>"`.
@@ -236,8 +248,16 @@ impl HtmlRenderer {
     }
 
     fn render_list(&self, n: &ListNode, w: &mut dyn Write) -> io::Result<()> {
+        self.render_list_nested(n, w, true)
+    }
+
+    fn render_list_nested(&self, n: &ListNode, w: &mut dyn Write, top: bool) -> io::Result<()> {
         let tag = if n.ordered { "ol" } else { "ul" };
+        if top && self.list_margin >= 0 {
+            writeln!(w, "<{tag} style=\"margin-left:{}em\">", self.list_margin)?;
+        } else {
             writeln!(w, "<{tag}>")?;
+        }
         for item in &n.items {
             self.render_list_item(item, w)?;
         }
@@ -267,7 +287,7 @@ impl HtmlRenderer {
         self.render_inlines(&n.children, w)?;
         if let Some(sublist) = &n.sublist {
             w.write_all(b"\n")?;
-            self.render_list(sublist, w)?;
+            self.render_list_nested(sublist, w, false)?;
         }
         w.write_all(b"</li>\n")
     }