feat(chat): share a chat by link — anonymous read-only /shared/<token> page, share/unshare

tests/unit/test_shared_header.py

@@ -424,7 +424,12 @@ def test_init_shared_header_rewrites_sign_in_next_to_current_page() -> None:
     """Phase 34 task 02: initSharedHeader points #sign-in-link at
     /login.html?next=<current pathname> (default "/") — the admin lands
     back on the page they signed in from. The page markup keeps its own
-    static ?next= as the no-JS fallback."""
+    static ?next= as the no-JS fallback.
+
+    Phase 51 (owner-locked 2026-08-29, TODO.md L6): the ONE exception —
+    the NESTED /shared/<token> page rewrites to the APP ROOT ("/")
+    instead of the shared URL: a guest signing in from a shared page
+    returns to the app root, not to a public link."""
     js = _text(HEADER_JS)
     fn = js.find("function initSharedHeader")
     assert fn != -1
@@ -432,7 +437,10 @@ def test_init_shared_header_rewrites_sign_in_next_to_current_page() -> None:
     # raw pathname (always a query-safe "/…" string — never "//", and ?
     # # / spaces stay percent-encoded inside it; login.js safeNext
     # re-validates), same shape as the static markup fallbacks
-    assert '"/login.html?next=" + (window.location.pathname || "/")' in body
+    assert 'const nextPath = window.location.pathname || "/";' in body
+    assert 'link.href = "/login.html?next=" + signInNext;' in body
+    # the shared-page exception: /shared/<token> → the app root
+    assert 'nextPath.startsWith("/shared/") ? "/" : nextPath' in body
 
 
 # ---------- phase 34 task 01: the steering controls move to the module ----------