feat: unified, editable frontmatter editor in publish modal

- Preset fields (from settings) are now seeded as editable/deletable rows in
  the modal alongside custom ones, using one consistent row style.
- The note's own merged frontmatter is shown as read-only (disabled) rows at
  the top of the list (only when 'Merge document frontmatter' is on, and only
  for keys not overridden by an editable row).
- Drops the separate read-only 'resolved' text preview; the rows are the
  WYSIWYG result.

Adds pure, unit-tested composeFrontmatter()/docFrontmatterToPairs() and removes
the superseded resolveFrontmatter(). Editable rows are authoritative: on a key
collision they override the merged document field. Presets are deep-copied so
editing rows never mutates saved settings.

Co-Authored-By: Claude

styles.css

@@ -1,2 +1,17 @@
-.jekyll-publish-row { display: flex; gap: 8px; margin-bottom: 6px; }
+.jekyll-publish-row { display: flex; gap: 8px; margin-bottom: 6px; align-items: center; }
 .jekyll-publish-row input { flex: 1; }
+
+/* Read-only document frontmatter fields shown when "Merge document
+   frontmatter" is on — visually muted to signal they cannot be edited here. */
+.jekyll-publish-row input.jekyll-publish-doc {
+  opacity: 0.65;
+  cursor: not-allowed;
+}
+
+/* Delete button on an editable property row. */
+.jekyll-publish-del {
+  flex: 0 0 auto;
+  width: 28px;
+  padding: 0;
+  line-height: 1;
+}