Put the selected chain in the page address

- Opening ?chain=testnet or ?chain=testnet4 selects that list, so it can
  be shared as a link; an unknown value falls back to Bitcoin.
- The chain buttons update the address and Back/Forward follow it.
- Bitcoin stays the default and keeps the plain address.

index.html

@@ -510,6 +510,20 @@ function getChain() {
   return btn ? btn.dataset.c : 'bitcoin';
 }
 
+// The selected chain lives in the address (?chain=testnet4), so a chain's list
+// can be shared as a link. Bitcoin is the default and keeps the plain address.
+function setChain(chain) {
+  const buttons = [...document.querySelectorAll('#chainSeg button')];
+  const target = buttons.find(b => b.dataset.c === chain && b.style.display !== 'none');
+  if (!target) return false;
+  buttons.forEach(b => b.setAttribute('aria-pressed', b === target ? 'true' : 'false'));
+  return true;
+}
+
+function chainFromUrl() {
+  return new URLSearchParams(window.location.search).get('chain') || 'bitcoin';
+}
+
 function getFiltered() {
   return allServers
     .filter(d => (d.url + " " + (d.info||"")).toLowerCase().includes(q.toLowerCase()));
@@ -609,8 +623,18 @@ document.getElementById('filter').addEventListener('input', e => {
 document.getElementById('chainSeg').addEventListener('click', e => {
   const b = e.target.closest('button');
   if (!b) return;
-  [...e.currentTarget.children].forEach(x => x.setAttribute('aria-pressed', 'false'));
-  b.setAttribute('aria-pressed', 'true');
+  setChain(b.dataset.c);
+  const url = new URL(window.location.href);
+  if (b.dataset.c === 'bitcoin') url.searchParams.delete('chain');
+  else url.searchParams.set('chain', b.dataset.c);
+  history.pushState(null, '', url);
+  fetchData();
+  fetchMinAmount();
+});
+
+// Browser Back/Forward: follow the address, an unknown chain falls back to Bitcoin.
+window.addEventListener('popstate', () => {
+  if (!setChain(chainFromUrl())) setChain('bitcoin');
   fetchData();
   fetchMinAmount();
 });
@@ -760,6 +784,7 @@ document.getElementById('url').addEventListener('keydown', e => {
 });
 
 (async function init() {
+  setChain(chainFromUrl());
   await Promise.all([fetchData(), fetchMinAmount()]);
   const blob = new Blob([JSON.stringify(allServers, null, 2)], { type: 'application/json' });
   document.getElementById('download').href = URL.createObjectURL(blob);