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

frontend/assets/header.js

@@ -106,9 +106,20 @@ export async function initSharedHeader() {
   // a query-safe "/…" string (never "//"; ? # and spaces stay
   // percent-encoded in it), so it rides in next= as-is — the same shape
   // the static fallbacks use (login.js safeNext re-validates it).
+  //
+  // Phase 51 (owner-locked 2026-08-29, TODO.md L6): the ONE exception —
+  // on the NESTED /shared/<token> page "where you were" is a public
+  // link, not a place in the app: the rewrite stays at the APP ROOT
+  // ("/"), so a guest signing in from a shared page lands in the chat
+  // (the static ?next=/ fallback in shared.html matches — the rewrite
+  // only ever KEEPS it there). A signed-in admin on the shared page
+  // never sees the link (hidden = admin), so this only shapes the
+  // guest experience.
+  const nextPath = window.location.pathname || "/";
+  const signInNext = nextPath.startsWith("/shared/") ? "/" : nextPath;
   document.querySelectorAll(".sign-in-link").forEach(link => {
     link.hidden = admin;
-    link.href = "/login.html?next=" + (window.location.pathname || "/");
+    link.href = "/login.html?next=" + signInNext;
   });
   document.querySelectorAll(".sign-out-btn").forEach(btn => { btn.hidden = !admin; });
   const navSources = document.querySelector("#nav-sources");