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

@@ -894,13 +894,16 @@ 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>
-              <button
-                onClick={reRegister}
-                disabled={ddnsRegistering}
-                className="px-3 py-1.5 text-xs font-medium rounded border border-blue-300 text-blue-700 hover:bg-blue-50 disabled:opacity-50"
-              >
-                {ddnsRegistering ? 'Registering…' : 'Re-register with pic.ngo'}
-              </button>
+              <div className="flex items-center gap-3">
+                <button
+                  onClick={reRegister}
+                  disabled={ddnsRegistering}
+                  className="px-3 py-1.5 text-xs font-medium rounded border border-blue-300 text-blue-700 hover:bg-blue-50 disabled:opacity-50"
+                >
+                  {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' && (