fix: clarify Re-register button purpose with inline hint

Add a short label explaining the button is for DDNS recovery (when the
DDNS server lost your record), not routine IP updates.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

webui/src/pages/Settings.jsx

@@ -894,6 +894,7 @@ function Settings() {
                 Your cell is registered as <span className="font-mono font-semibold">{domainName || `${identity.cell_name}.pic.ngo`}</span> on pic.ngo.
                 Change the Cell Name above to update this subdomain.
               </div>
+              <div className="flex items-center gap-3">
                 <button
                   onClick={reRegister}
                   disabled={ddnsRegistering}
@@ -901,6 +902,8 @@ function Settings() {
                 >
                   {ddnsRegistering ? 'Registering…' : 'Re-register with pic.ngo'}
                 </button>
+                <span className="text-xs text-gray-400">Use if DDNS lost your record — sends current public IP</span>
+              </div>
             </div>
           )}
           {domainMode === 'cloudflare' && (