fix(themes): Define the chart token in the high-contrast dark theme (#69)

contrast.test.ts requires every plain colour token in high-contrast dark; #63 added
--chart-series to the dark and light themes only.

Claude-Session: https://claude.ai/code/session_01VUYsdXA2nkF5v6zsC93dpn

src/styles/tokens.css

@@ -645,6 +645,8 @@ html {
   --link: #a7c5ff;
   --link-hover: #d0dffe;
 
+  --chart-series: #ff9b9f;
+
   --danger: #ff9b9f;
   --warning: #f2c55c;
   --success: #8ec39d;