fix: add --bg-surface CSS variable to tokens.css

Fills the gap between --bg-card (#1a1a2e) and --bg-elevated (#1e1e3a)
for JS modules that reference this token.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

public/css/tokens.css

@@ -3,6 +3,7 @@
   --bg-base: #0a0a14;
   --bg-card: #1a1a2e;
   --bg-elevated: #1e1e3a;
+  --bg-surface: #1c1c34;
   --bg-hover: #2a2a4a;
   --bg-panel: #0f0f1e;
   --bg-inset: #0a1420;