fix: add --text-inverse CSS variable for badge icon contrast

The design system doesn't define --text-inverse yet, so SVG icons
on colored badge backgrounds (trace, tap, status) had no fill color.
Define it in index.css as #fff until the DS adds it natively.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/index.css

@@ -62,6 +62,8 @@
 :root {
   /* Light mode: #766A5E on #FFFFFF = 4.5:1 (was #9C9184 = 3.0:1) */
   --text-muted: #766A5E;
+  /* White text on colored badge backgrounds (not in DS yet) */
+  --text-inverse: #fff;
 }
 
 [data-theme="dark"] {