polish: responsive grid, radial gradient background, better bento card scaling

- Dashboard grid is now responsive: 1 col mobile, 2 cols tablet, 4 cols desktop
- BentoCard span classes converted to responsive variants
- Subtle radial gradient on main background for depth (as per design system §7.7)
- All production builds passing

src/index.css

@@ -103,6 +103,7 @@ body,
   width: 100%;
   overflow: hidden;
   background: var(--color-bg-deep);
+  background: radial-gradient(ellipse at 50% 0%, #141e33 0%, var(--color-bg-deep) 70%);
   color: var(--color-text-primary);
   font-family: var(--font-sans);
   font-size: 14px;