fix(ui): document viewer back button returns to the page you came from (chat or sources)

frontend/assets/app.js

@@ -65,14 +65,21 @@ const reducedMotion =
   typeof matchMedia === "function" && matchMedia("(prefers-reduced-motion: reduce)").matches;
 const SCROLL = reducedMotion ? "auto" : "smooth";
 
-/* ---------- document viewer link (phase 10) ----------
- * Every cited document opens in the viewer, in a NEW tab. Both query
+/* ---------- document viewer link (phase 10; phase 13 adds `back`) ----------
+ * Every cited document opens in the viewer, in a NEW tab. All query
  * values are percent-encoded: real paths contain slashes and sometimes
- * spaces, which would otherwise corrupt the query string. (The renderer
+ * spaces, which would otherwise corrupt the query string. `back` tells the
+ * viewer which page to return to when its back button is clicked — the
+ * chips live in the chat, so chat passes "/" (the viewer validates it:
+ * only same-origin relative URLs are honored; Sources links omit it and
+ * get the viewer's /sources.html default). (The renderer
  * renderMarkdown/escapeHtml now lives in assets/markdown.js — a classic
  * script loaded by index.html and document.html before these modules.) */
-export function documentUrl(source, path) {
-  return "/document.html?source=" + encodeURIComponent(source) + "&path=" + encodeURIComponent(path);
+export function documentUrl(source, path, back = "/") {
+  let url =
+    "/document.html?source=" + encodeURIComponent(source) + "&path=" + encodeURIComponent(path);
+  if (back) url += "&back=" + encodeURIComponent(back);
+  return url;
 }
 
 /* ---------- avatar glyphs (phase 08: emoji-free chrome) ----------
@@ -291,7 +298,7 @@ function appendSources(wrap, sources) {
     const chip = document.createElement("a");
     chip.className = "source-chip";
     chip.setAttribute("role", "listitem");
-    chip.href = documentUrl(s.source, s.path);
+    chip.href = documentUrl(s.source, s.path, "/"); // back → the chat page
     chip.target = "_blank"; // open the full document in a new tab
     chip.rel = "noopener";
     chip.textContent = label;