style(editor): align footnote marker and center task checkbox

Remove top margin from the first paragraph in footnote definitions so the
marker aligns with the first line of text. Adjust the task‑list label to use
the editor line‑height variable for its height and center the checkbox
vertically, keeping it in line with the item’s first text line.

apps/client/src/features/editor/components/footnote/footnote.module.css

@@ -104,6 +104,19 @@
   min-width: 0;
 }
 
+/* The inner editable paragraph inherits `.ProseMirror p { margin: 0.5em 0 }`,
+   which pushes the first text line ~0.5em below the "N." marker (aligned to
+   flex-start), making the number float above the text. Drop the outer margins
+   so the marker and the first line share the same top edge — same approach
+   used for callouts in core.css. */
+.definitionContent > :first-child {
+  margin-top: 0;
+}
+
+.definitionContent > :last-child {
+  margin-bottom: 0;
+}
+
 .backLink {
   flex: 0 0 auto;
   cursor: pointer;