feat: align design tokens with Vectry unified design language

- Change background from #0a0a0f to #0a0a0a (matching vectry.tech + AgentLens)
- Remove unused --accent-cyan and --shadow-card tokens
- Add shared Vectry design aliases (surface-page, surface-card, border-default, radius-card, etc.)
- Fix hardcoded #0a0a0f in layout.tsx, history/page.tsx, and mermaid-diagram.tsx

apps/web/src/app/globals.css

@@ -2,7 +2,7 @@
 @plugin "@tailwindcss/typography";
 
 :root {
-  --background: #0a0a0f;
+  --background: #0a0a0a;
   --surface: rgba(255, 255, 255, 0.03);
   --surface-hover: rgba(255, 255, 255, 0.06);
   --border: rgba(255, 255, 255, 0.08);
@@ -15,13 +15,21 @@
   --accent-blue: #3b82f6;
   --accent-indigo: #6366f1;
   --accent-purple: #9333ea;
-  --accent-cyan: #06b6d4;
   
   --gradient-primary: linear-gradient(135deg, #3b82f6 0%, #6366f1 50%, #9333ea 100%);
   --gradient-subtle: linear-gradient(135deg, rgba(59, 130, 246, 0.1) 0%, rgba(147, 51, 234, 0.1) 100%);
   
   --shadow-glow: 0 0 40px rgba(59, 130, 246, 0.3);
-  --shadow-card: 0 4px 24px rgba(0, 0, 0, 0.4);
+
+  /* Shared Vectry design language aliases */
+  --surface-page: var(--background);
+  --surface-card: var(--surface);
+  --surface-card-hover: var(--surface-hover);
+  --border-default: var(--border);
+  --border-subtle: rgba(255, 255, 255, 0.04);
+  --radius-card: 1rem;
+  --radius-button: 0.5rem;
+  --radius-icon: 0.75rem;
 }
 
 * {