fix(whatsnew): decode HTML entities and trim em-dash detail in fallback bullets

The Features-fallback showed raw changelog text, so a commit subject like
'branded URL shortener — /s/<slug> with OG injection' surfaced two problems
in the admin banner:
- release-please escapes <slug> to &lt;slug&gt;; React renders the literal
  entity, so the banner read '/s/&lt;slug&gt;'. Decode the entities
  (&lt; &gt; &amp; &quot; &#39;), &amp; last to avoid double-decoding.
- the technical tail leaked into a user-facing highlight. Drop a trailing
  '— detail' clause (em dash only, so 'mark-paid' is untouched) so the bullet
  reads as the headline 'branded URL shortener'.

Only affects the deterministic fallback; curated <!-- whatsnew --> blocks are
unchanged.

backend/src/utils/whatsNew.js

@@ -14,14 +14,35 @@
 
 const MAX_BULLETS = 8;
 
+/**
+ * Decode the handful of HTML entities release-please escapes into changelog
+ * text (a raw "/s/<slug>" in a commit subject lands as "/s/&lt;slug&gt;").
+ * Without this the banner shows the literal entity, since React renders text
+ * nodes verbatim. `&amp;` is decoded last so "&amp;lt;" stays "&lt;".
+ */
+function decodeEntities(s) {
+  return s
+    .replace(/&lt;/g, '<')
+    .replace(/&gt;/g, '>')
+    .replace(/&quot;/g, '"')
+    .replace(/&#0*39;|&#x0*27;|&apos;/gi, "'")
+    .replace(/&amp;/g, '&');
+}
+
 /** Strip list markers, conventional-commit scope, and trailing PR/sha links. */
 function cleanBullet(line) {
-  return line
+  return decodeEntities(line
     .replace(/^\s*[-*]\s+/, '')                  // "- " / "* " marker
     .replace(/^\*\*([^:*]+):\*\*\s*/, '')         // "**scope:** " prefix
     .replace(/\s*\(\[[^\]]*\]\([^)]*\)\)/g, '')   // " ([#41](url))" / " ([sha](url))"
     .replace(/\s*\(#\d+\)/g, '')                  // bare " (#41)"
-    .replace(/`/g, '')
+    .replace(/`/g, ''))
+    // Drop a trailing "— implementation detail" clause so a release highlight
+    // reads as the headline ("branded URL shortener"), not the commit subject
+    // ("branded URL shortener — /s/<slug> with OG injection"). Em dash only, so
+    // hyphenated words ("mark-paid") are untouched. Skipped if it would empty
+    // the bullet (i.e. nothing before the dash).
+    .replace(/^(.+?\S)\s+—\s+.*$/, '$1')
     .replace(/\s+/g, ' ')
     .trim();
 }