fix(05): add explicit radius scale tokens for Tailwind v4 cascade

Tailwind v4 does not derive --radius-xs through --radius-3xl from --radius.
The named scale is independent, so Cards (rounded-xl), Buttons (rounded-md),
etc. retained non-zero border-radius despite --radius: 0. Adding all scale
tokens as 0 ensures complete sharp-corner cascade.

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

src/index.css

@@ -63,6 +63,13 @@
   --color-investment-fill: oklch(0.68 0.22 285);
 
   --radius: 0;
+  --radius-xs: 0;
+  --radius-sm: 0;
+  --radius-md: 0;
+  --radius-lg: 0;
+  --radius-xl: 0;
+  --radius-2xl: 0;
+  --radius-3xl: 0;
 
   /* Collapsible animation */
   --animate-collapsible-open: collapsible-open 200ms ease-out;