style(board): landscape sheets, larger sizing, and web fonts

Render A-series sheets in landscape (√2:1), size them off a single
--sheet-base variable so A2/A3/A4 keep exact ISO ratios while leaving room
for labels (which now wrap). Load Playfair Display + Cutive Mono via the
coollabs fonts API: Playfair Display for headings, Cutive Mono for the board.

app/assets/css/main.css

@@ -0,0 +1,10 @@
+/* Headings use Playfair Display (loaded via the coollabs fonts API in
+   nuxt.config). The board supplies its own Cutive Mono for section labels. */
+h1,
+h2,
+h3,
+h4,
+h5,
+h6 {
+  font-family: 'Playfair Display', Georgia, serif;
+}