Let the AS page's prefix lists fold (closes #155)

The IPv4 and IPv6 prefix lists on the AS page are each a <details>
element. Its <summary> holds the list's heading and count, so a click on
the heading folds or unfolds the list without a script. Both lists start
open. The peer list is unchanged.

The summary keeps the look of the other section headings. It hides the
browser's own marker, which not every browser shows on a flex element,
and draws a triangle before the heading that points down while the list
is open.

Model: opus-5-5

internal/templates/as_detail.html

@@ -126,6 +126,26 @@
             margin: 0;
             color: #2c3e50;
         }
+        /* A click on a prefix list's heading folds or unfolds the list. Not
+           every browser shows its own marker on a flex element, so the page
+           hides it and draws a triangle before the heading. The triangle
+           points down while the list is open. */
+        summary.prefix-header {
+            cursor: pointer;
+            list-style: none;
+        }
+        summary.prefix-header::-webkit-details-marker {
+            display: none;
+        }
+        summary.prefix-header h2::before {
+            content: "\25B8";
+            display: inline-block;
+            margin-right: 10px;
+            transition: transform 0.2s;
+        }
+        details[open] > summary.prefix-header h2::before {
+            transform: rotate(90deg);
+        }
         .prefix-count {
             background: #e74c3c;
             color: white;
@@ -242,11 +262,11 @@
         </div>
 
         {{if .IPv4Prefixes}}
-        <div class="prefix-section">
-            <div class="prefix-header">
+        <details class="prefix-section" open>
+            <summary class="prefix-header">
                 <h2>IPv4 Prefixes</h2>
                 <span class="prefix-count">{{.IPv4Count}}</span>
-            </div>
+            </summary>
             <table class="prefix-table">
                 <thead>
                     <tr>
@@ -267,15 +287,15 @@
                     {{end}}
                 </tbody>
             </table>
-        </div>
+        </details>
         {{end}}
 
         {{if .IPv6Prefixes}}
-        <div class="prefix-section">
-            <div class="prefix-header">
+        <details class="prefix-section" open>
+            <summary class="prefix-header">
                 <h2>IPv6 Prefixes</h2>
                 <span class="prefix-count">{{.IPv6Count}}</span>
-            </div>
+            </summary>
             <table class="prefix-table">
                 <thead>
                     <tr>
@@ -296,7 +316,7 @@
                     {{end}}
                 </tbody>
             </table>
-        </div>
+        </details>
         {{end}}
 
         {{if eq .TotalCount 0}}