fix: improve pill contrast by using WCAG contrast ratios on composited backgrounds

Pills with semi-transparent backgrounds were using raw color luminance to pick
text color, ignoring the page background showing through. This caused unreadable
text on dark themes for mid-luminance colors like orange. Now composites the
rgba background over the actual page bg (dark/light) before computing WCAG
contrast ratios, and centralizes the logic in a shared color-contrast utility.

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

ui/src/components/IssuesList.tsx

@@ -1,5 +1,6 @@
 import { useEffect, useMemo, useState, useCallback, useRef } from "react";
 import { useQuery } from "@tanstack/react-query";
+import { pickTextColorForPillBg } from "@/lib/color-contrast";
 import { useDialog } from "../context/DialogContext";
 import { useCompany } from "../context/CompanyContext";
 import { issuesApi } from "../api/issues";
@@ -719,7 +720,7 @@ export function IssuesList({
                               className="inline-flex items-center rounded-full border px-1.5 py-0.5 text-[10px] font-medium"
                               style={{
                                 borderColor: label.color,
-                                color: label.color,
+                                color: pickTextColorForPillBg(label.color, 0.12),
                                 backgroundColor: `${label.color}1f`,
                               }}
                             >