Sign out through the proxy when the proxy signed you in

Sign out cleared ipx's cookies and showed its password page, while
Cloudflare Access still vouched for the person: nothing was signed out,
and the page looked like the wrong login. /api/me now says, for someone
the proxy signed in, where to go instead ([web] sign_out_url, which is
/cdn-cgi/access/logout behind Access), and /login sends anyone the proxy
vouches for on to their feeds. The header check both use is one function.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TAC7sLVqfKmY6rsTLXzNgk

web/index.html

@@ -1869,7 +1869,9 @@ function opmlModal(){
 async function scanAll(){ toast('Scanning all feeds…'); await api('/api/fetch',{method:'POST',body:JSON.stringify({force:true})}); }
 $('#scanAll').onclick=scanAll;
 $('#prefs').onclick=prefsModal;
-$('#signout').onclick=async()=>{ await api('/api/logout',{method:'POST'}); location.href='/login'; };
+// Someone the proxy signed in is signed out by the proxy: ipx's own sign-out cannot stick while
+// the proxy still vouches for them. /api/me says where, when that is the case.
+$('#signout').onclick=async()=>{ await api('/api/logout',{method:'POST'}); location.href=S.me?.sign_out||'/login'; };
 api('/api/me').then(u=>{
   S.me=u;
   $('#who').textContent=u.name+(u.admin?' · admin':'');