Fix IPv6 prefix length links to use separate /prefixlength6/<len> route

The prefix length links for IPv6 prefixes were incorrectly pointing to
/prefixlength/<len> which would show IPv4 prefixes. Added a new route
/prefixlength6/<len> specifically for IPv6 prefixes and updated the
template to use the correct URL based on whether displaying IPv4 or IPv6
prefix distributions.

Also refactored handlePrefixLength to explicitly handle only IPv4 prefixes
and created handlePrefixLength6 for IPv6 prefixes, removing the ambiguous
auto-detection based on mask length value.

internal/templates/status.html

@@ -236,12 +236,16 @@
             // Sort by mask length
             distribution.sort((a, b) => a.mask_length - b.mask_length);
             
+            // Determine the URL path based on whether this is IPv4 or IPv6
+            const isIPv6 = elementId.includes('ipv6');
+            const urlPath = isIPv6 ? '/prefixlength6/' : '/prefixlength/';
+            
             distribution.forEach(item => {
                 const metric = document.createElement('div');
                 metric.className = 'metric';
                 metric.innerHTML = `
                     <span class="metric-label">/${item.mask_length}</span>
-                    <a href="/prefixlength/${item.mask_length}" class="metric-value metric-link">${formatNumber(item.count)}</a>
+                    <a href="${urlPath}${item.mask_length}" class="metric-value metric-link">${formatNumber(item.count)}</a>
                 `;
                 container.appendChild(metric);
             });