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

tests/ui/global-setup.js

@@ -37,6 +37,10 @@ enabled = false
 enabled = true
 bind = "127.0.0.1:8791"
 token = "${TOKEN}"
+# The proxy path, for tests that send the header themselves: the daemon sees them at 127.0.0.1.
+trusted_header = "X-Test-User"
+trusted_proxies = ["127.0.0.1"]
+sign_out_url = "/signed-out-by-the-proxy"
 
 [feeds.test-show]
 url = "http://127.0.0.1:8792/show.xml"