fix(ui): give the source breakdown disclosure a visible focus ring

Its summary set focus-visible:outline-none, which left a keyboard
reader with no visible focus there. It now shares the settings groups'
focus rule and chevron.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/client/components/Colophon.tsx

@@ -290,8 +290,15 @@ export function Colophon({
           )}
 
           {shownStale.some((s) => s.sources && s.sources.length > 0) && (
-            <details className="mt-2 text-faint">
-              <summary className="cursor-pointer hover:text-muted focus-visible:outline-none">
+            <details className="disclosure mt-2 text-faint">
+              <summary className="flex w-fit items-center gap-1.5 hover:text-muted">
+                <svg
+                  aria-hidden
+                  viewBox="0 0 16 16"
+                  className="settings-chevron size-2 shrink-0"
+                >
+                  <path d="M5 2.5l6 5.5-6 5.5z" fill="currentColor" />
+                </svg>
                 Source breakdown
               </summary>
               <ul className="mt-1.5 space-y-1 border-l border-hairline pl-3">

src/client/styles.css

@@ -433,16 +433,19 @@ body {
  * `[tabindex]`, so the focus rule above does not reach it and the one control
  * that opens a group would have no visible focus at all.
  */
-.settings-group > summary {
+.settings-group > summary,
+.disclosure > summary {
   list-style: none;
   cursor: pointer;
 }
 
-.settings-group > summary::-webkit-details-marker {
+.settings-group > summary::-webkit-details-marker,
+.disclosure > summary::-webkit-details-marker {
   display: none;
 }
 
-.settings-group > summary:focus-visible {
+.settings-group > summary:focus-visible,
+.disclosure > summary:focus-visible {
   outline: 2px solid var(--color-near);
   outline-offset: 2px;
   border-radius: 3px;
@@ -461,7 +464,8 @@ body {
   transition: transform 150ms ease;
 }
 
-.settings-group[open] > summary .settings-chevron {
+.settings-group[open] > summary .settings-chevron,
+.disclosure[open] > summary .settings-chevron {
   transform: rotate(90deg);
 }