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/prefix_length.html

@@ -93,7 +93,7 @@
         <tbody>
             {{ range .Prefixes }}
             <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 class="age">{{ .Age }}</td>
                 <td>
                     <a href="/as/{{ .OriginASN }}" class="as-link">