host: discoverable log in / log out footer link

Login had no visible entry point — you could only reach it by hitting a guard.
Add an auth footer the pages splice in: "log in" when logged out, "signed in
as <user> · log out" when logged in.

- host/auth-footer: SX fragment reading the session principal; guards a
  session-less request so it's safe to call anywhere.
- GET /logout added alongside POST so the footer link is a plain <a> (logout
  is low-harm; GET is acceptable). Clears the session, redirects home.
- home and post pages splice (host/auth-footer req) into their footer.

Tests: home + post footers show a login link when anonymous; GET /logout ->
303. 221/221. Verified live: anonymous shows "log in"; logged in shows
"signed in as admin · log out"; /logout reverts it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/host/blog.sx

@@ -115,7 +115,9 @@
                     " · "
                     (a :href (unquote (str "/" slug "/edit")) "edit")
                     " · "
-                    (a :href "/" "all posts"))))))
+                    (a :href "/" "all posts")
+                    " · "
+                    (unquote (host/auth-footer req)))))))
           (dream-html-status 404
             (host/blog--page "Not found"
               (quasiquote
@@ -140,7 +142,9 @@
               (quasiquote
                 (div (h1 "Posts")
                   (unquote listing)
-                  (p (a :href "/new" "+ New post")))))))))))
+                  (p (a :href "/new" "+ New post"))
+                  (p :style "margin-top:2em;font-size:0.9em;opacity:0.8"
+                    (unquote (host/auth-footer req))))))))))))
 
 (define host/blog-index (fn (req) (host/ok (host/blog-list))))