fix: book transcript browser navigation

app/(app)/transcriptions/book/[book]/book-browser.tsx

@@ -2,6 +2,7 @@
 
 import { useState, useMemo, useRef } from "react";
 import Link from "next/link";
+import { usePathname, useSearchParams } from "next/navigation";
 import { Card, CardContent } from "@/components/ui/card";
 import { Badge } from "@/components/ui/badge";
 import { buttonVariants } from "@/components/ui/button";
@@ -384,9 +385,18 @@ export function TranscriptionBookBrowser({
 
   const flat = useMemo(() => flattenChapters(sections), [sections]);
 
-  const [activeKeyRaw, setActiveKey] = useState<string>(
-    flat.length > 0 ? flat[0].chapter.key : "",
-  );
+  // The active chapter lives in the URL (?chapter=<key>) so it's bookmarkable
+  // and survives back-navigation: opening a lecture then pressing Back returns
+  // to the chapter you were on rather than resetting to the first one. We seed
+  // local state from the URL on mount and keep the URL in sync on selection.
+  const pathname = usePathname();
+  const searchParams = useSearchParams();
+
+  const [activeKeyRaw, setActiveKey] = useState<string>(() => {
+    const fromUrl = searchParams.get("chapter");
+    if (fromUrl && flat.find((f) => f.chapter.key === fromUrl)) return fromUrl;
+    return flat.length > 0 ? flat[0].chapter.key : "";
+  });
 
   // Derive a safe active chapter key (the data could change underneath us).
   const activeKey = useMemo(() => {
@@ -399,6 +409,11 @@ export function TranscriptionBookBrowser({
 
   const handleSelect = (key: string) => {
     setActiveKey(key);
+    // Reflect the choice in the URL without a new history entry (so Back leaves
+    // the page rather than cycling chapters) or a server round-trip.
+    const params = new URLSearchParams(Array.from(searchParams.entries()));
+    params.set("chapter", key);
+    window.history.replaceState(null, "", `${pathname}?${params.toString()}`);
     contentRef.current?.scrollIntoView({ behavior: "smooth", block: "start" });
   };