Fix prefix URL routing for encoded CIDR notation

Change route from wildcard /prefix/* to explicit /prefix/{prefix}/{len}
to properly handle URL-encoded IPv6 addresses with CIDR notation.

- Separate prefix and length into individual path parameters
- Add prefixURL template function for generating correct links
- Remove url.QueryUnescape from handlers (chi handles decoding)

internal/templates/as_detail.html

@@ -182,7 +182,7 @@
                 <tbody>
                     {{range .IPv4Prefixes}}
                     <tr>
-                        <td><a href="/prefix/{{.Prefix | urlEncode}}" class="prefix-link">{{.Prefix}}</a></td>
+                        <td><a href="{{.Prefix | prefixURL}}" class="prefix-link">{{.Prefix}}</a></td>
                         <td>/{{.MaskLength}}</td>
                         <td>{{.LastUpdated.Format "2006-01-02 15:04:05"}}</td>
                         <td class="age">{{.LastUpdated | timeSince}}</td>
@@ -211,7 +211,7 @@
                 <tbody>
                     {{range .IPv6Prefixes}}
                     <tr>
-                        <td><a href="/prefix/{{.Prefix | urlEncode}}" class="prefix-link">{{.Prefix}}</a></td>
+                        <td><a href="{{.Prefix | prefixURL}}" class="prefix-link">{{.Prefix}}</a></td>
                         <td>/{{.MaskLength}}</td>
                         <td>{{.LastUpdated.Format "2006-01-02 15:04:05"}}</td>
                         <td class="age">{{.LastUpdated | timeSince}}</td>