Style title screen with gradient, bordered frame, and New Game button

Gradient background spans the full window on body; the 1rem-inset frame
provides the black border and rounded corners as a transparent overlay.
"Black" and "7" are typographically separated, with "7" oversized.
New Game button is full-width, light blue, pinned to the bottom.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/index.css

@@ -0,0 +1,7 @@
+*, *::before, *::after { box-sizing: border-box; }
+html, body, #root { margin: 0; height: 100%; }
+body {
+  font-family: sans-serif;
+  background: linear-gradient(to bottom, #a8d5e8, #4a9e6a);
+  min-height: 100vh;
+}