host: browser auth redirects to login (no more raw JSON 401), with return-to

Clicking "edit" while logged out returned a raw JSON 401
{"ok":false,"error":"unauthorized"} — a dead end in the browser. HTML routes
now redirect to a usable login page and return you afterwards.

- host/require-login: browser-shaped guard. Same session-or-bearer check as
  host/require-user, but on failure REDIRECTS to /login?next=<path> instead of
  JSON 401. (host/require-user stays for JSON/API routes.)
- host/-principal-of: shared session-then-bearer resolution.
- login honours ?next=: GET /login renders a hidden next field; POST /login
  redirects there on success and re-renders the form (with next) on failure.
- host/-safe-next: only same-site absolute paths are honoured — //evil.com and
  http://… fall back to "/", closing the open-redirect.
- blog: host/blog--protect-html (require-login) guards the browser routes —
  POST /new, GET/POST /:slug/edit; the JSON /posts routes keep host/require-user.

Do we need login? Yes — it's the write/edit auth boundary; without it anyone
could edit or delete posts. The bug was the dead-end 401, not the gate. Now
logged-out edit -> login -> back to edit is a clean flow.

Tests: blog no-auth write routes assert 303 + Location /login(+next); session
suite gains next round-trip + open-redirect-guard cases. 218/218.
Verified live: /welcome/edit logged out -> 303 /login?next=/welcome/edit;
login -> 303 back to /welcome/edit -> 200.

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

lib/host/tests/blog.sx

@@ -70,10 +70,14 @@
 (host/blog-use-store! (persist/open))
 
 ;; -- editor form ingest (form-urlencoded, the editor's submit shape) --
-(host-bl-test "form ingest no auth -> 401"
+(host-bl-test "form ingest no auth -> redirect to login"
   (dream-status (host-bl-wapp (host-bl-send "POST" "/new" nil
     "application/x-www-form-urlencoded" "title=X")))
-  401)
+  303)
+(host-bl-test "form ingest no auth Location is /login"
+  (contains? (dream-resp-header (host-bl-wapp (host-bl-send "POST" "/new" nil
+    "application/x-www-form-urlencoded" "title=X")) "location") "/login")
+  true)
 (host-bl-test "form ingest authed -> 303 redirect"
   (dream-status (host-bl-wapp (host-bl-send "POST" "/new" "Bearer good"
     "application/x-www-form-urlencoded"
@@ -164,17 +168,22 @@
   (dream-status (host-bl-wapp (host-bl-req "/my-first-post/"))) 200)
 
 ;; -- edit source (guarded GET form + guarded POST save) --
-(host-bl-test "edit form no auth -> 401"
-  (dream-status (host-bl-wapp (host-bl-send "GET" "/my-first-post/edit" nil "" ""))) 401)
+(host-bl-test "edit form no auth -> redirect to login"
+  (dream-status (host-bl-wapp (host-bl-send "GET" "/my-first-post/edit" nil "" ""))) 303)
+(host-bl-test "edit form no auth Location carries next=/…/edit"
+  (contains?
+    (dream-resp-header (host-bl-wapp (host-bl-send "GET" "/my-first-post/edit" nil "" "")) "location")
+    "/login?next=/my-first-post/edit")
+  true)
 (host-bl-test "edit form authed -> 200"
   (dream-status (host-bl-wapp (host-bl-send "GET" "/my-first-post/edit" "Bearer good" "" ""))) 200)
 (host-bl-test "edit form shows current source"
   (contains? (dream-resp-body (host-bl-wapp (host-bl-send "GET" "/my-first-post/edit" "Bearer good" "" "")))
     "(article")
   true)
-(host-bl-test "edit submit no auth -> 401"
+(host-bl-test "edit submit no auth -> redirect to login"
   (dream-status (host-bl-wapp (host-bl-send "POST" "/my-first-post/edit" nil
-    "application/x-www-form-urlencoded" "sx_content=(p+%22x%22)"))) 401)
+    "application/x-www-form-urlencoded" "sx_content=(p+%22x%22)"))) 303)
 (host-bl-test "edit submit authed -> 303"
   (dream-status (host-bl-wapp (host-bl-send "POST" "/my-first-post/edit" "Bearer good"
     "application/x-www-form-urlencoded"