fix: a columns section's first block starts flush with the column top

The rule that drops the space above it never applied to a Word document:
:is(p, h1, …, .toc) takes the class specificity of .toc, so the later
spacing-model rule tied at (0,4,0) and won on order — and the heading
opening a two-column section sat 3.5mm below the column beside it.
Zeroing the property both spacing models read settles it, and beats the
block's own inline value as a margin would not. Probed: LibreOffice
starts both columns level; a two-column page now matches it 45 lines
deep, where it diverged at line 8.

src/styles/editor.css

@@ -531,7 +531,11 @@
 .paper .tiptap .columns-node {
   column-fill: balance;
 }
+/* Its first block sits flush with the column top — the space above it is dropped there,
+   as at a page top (probed: LibreOffice starts both columns level). Zeroing the property
+   the two spacing models read beats the block's own inline value, which a margin wouldn't. */
 .paper .tiptap .columns-node > :first-child {
+  --space-before: 0 !important;
   margin-top: 0;
   padding-top: 0;
 }