fix: bookmarks.js silently destroyed a PDF's existing bookmarks when adding a new one

Add Bookmarks always rebuilt /Outlines from scratch containing only what the
user typed in the current session, never reading the PDF's own pre-existing
outline first. A user adding one bookmark to an already-bookmarked PDF (e.g.
a Word/InDesign export with a heading-based TOC) silently lost every existing
bookmark in the output, with zero warning.

Fixed by reading the source PDF's top-level /Outlines linked list at load
time (same resolution logic as split-by-bookmarks.js's readTopLevelOutline)
and pre-populating the bookmarks queue with it, so existing entries are
preserved unless the user explicitly removes them via the same Remove button
used for newly-added ones. Verified against the real vendored pdf-lib.js:
reproduced the original bug (2 existing bookmarks -> 1 after adding one new
one), then confirmed the fix (3 bookmarks survive), plus a no-existing-outline
PDF still loads with an empty queue and no throw.

Also reworded the success message from "N bookmark(s) added" to "N bookmark(s)
saved", since the count can now include preserved pre-existing entries, not
just ones typed this session.

Author: analytics429

bookmarks.js

@@ -102,13 +102,59 @@ function updateActions() {
   saveBtn.disabled = bookmarks.length === 0;
 }
 
+/**
+ * Reads the PDF's own top-level /Outlines linked list, if any, so pre-existing
+ * bookmarks aren't silently destroyed when the user adds a new one — addOutline()
+ * below always rebuilds /Outlines from scratch, so whatever isn't in `bookmarks`
+ * at save time is gone. Only direct [pageRef, ...] /Dest arrays are resolved
+ * (not named destinations via /Names, and not /A GoTo actions) — an entry that
+ * can't be resolved to a page is dropped rather than guessed at.
+ */
+async function readExistingBookmarks(doc) {
+  const { PDFName } = await getPdfLib();
+  const context = doc.context;
+  const outlinesRef = doc.catalog.get(PDFName.of("Outlines"));
+  if (!outlinesRef) return [];
+  const outlines = context.lookup(outlinesRef);
+  if (!outlines) return [];
+
+  const pageRefs = doc.getPages().map((p) => p.ref);
+  function resolvePage(dest) {
+    if (!dest || typeof dest.get !== "function") return -1;
+    try {
+      const pageRef = dest.get(0);
+      if (!pageRef) return -1;
+      return pageRefs.findIndex(
+        (r) => r.tag === pageRef.tag && r.objectNumber === pageRef.objectNumber
+      );
+    } catch (e) {
+      return -1;
+    }
+  }
+
+  const items = [];
+  let cur = outlines.get(PDFName.of("First"));
+  let guard = 0;
+  while (cur && guard++ < 10000) {
+    const item = context.lookup(cur);
+    if (!item) break;
+    const titleObj = item.get(PDFName.of("Title"));
+    const title = titleObj && titleObj.decodeText ? titleObj.decodeText() : "";
+    const dest = item.get(PDFName.of("Dest"));
+    const pageIndex = resolvePage(dest);
+    if (title && pageIndex >= 0) items.push({ title, page: pageIndex + 1 });
+    cur = item.get(PDFName.of("Next"));
+  }
+  return items;
+}
+
 async function loadFile(file) {
   try {
     const { PDFDocument } = await getPdfLib();
     const bytes = await file.arrayBuffer();
     const doc = await PDFDocument.load(bytes, { ignoreEncryption: true });
     loaded = { file, pageCount: doc.getPageCount() };
-    bookmarks = [];
+    bookmarks = await readExistingBookmarks(doc).catch(() => []);
     renderFile();
     renderBookmarks();
     updateActions();
@@ -291,7 +337,7 @@ saveBtn.addEventListener("click", async () => {
     resultEl.setAttribute("role", "status");
     resultEl.setAttribute("aria-live", "polite");
     resultEl.innerHTML = `
-      <span><strong>Done.</strong> ${count} bookmark${count === 1 ? "" : "s"} added —
+      <span><strong>Done.</strong> ${count} bookmark${count === 1 ? "" : "s"} saved —
       ${elapsedMs}ms, ${requestsDuring} network requests, entirely on this device.</span>
       <a class="btn btn-primary" href="${url}" download="${escapeHtml(fileName)}">Download ${escapeHtml(fileName)}</a>
     `;