feat(ui): browser back and forward step through the app

The app's place (page, open note, workspace view, zoomed bullet, on phones
list or note) goes into the browser history (lib/navHistory.ts), so the
back button of the mouse, the browser or Android steps back inside the
app: out of a zoom, to the previous note, to the previous page. Changes
from one action become one entry, the app's own choice of a first note
replaces the entry, and following a step adds nothing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/features/workspace/DocumentEditor.tsx

@@ -34,6 +34,7 @@ import { Presence } from '../../lib/docSync/transport';
 import { OutlinerTree, OutlinerTreeHandle } from './OutlinerTree';
 import { extractHashtags } from '../../lib/hashtags';
 import { moveSibling } from '../../lib/outlineList';
+import { reportNav, onNavStep, restoringNavPlace } from '../../lib/navHistory';
 
 export type DocSaveStatus = 'saved' | 'saving' | 'unsaved' | 'error';
 
@@ -260,7 +261,11 @@ export const DocumentEditor: React.FC<DocumentEditorProps> = ({
   const [loading, setLoading] = useState<boolean>(!initialDoc);
   const [error, setError] = useState<string | null>(null);
   const [showEmojiPicker, setShowEmojiPicker] = useState<boolean>(false);
-  const [zoomedBlockId, setZoomedBlockId] = useState<string | null>(null);
+  // Zoomed bullet; coming here by the browser's back/forward it is the zoom of that step
+  const [zoomedBlockId, setZoomedBlockId] = useState<string | null>(() => {
+    const place = restoringNavPlace();
+    return !inJournal && !inTagPage && place?.docId === docId ? place.zoom ?? null : null;
+  });
   const [showActionsMenu, setShowActionsMenu] = useState<boolean>(false);
   const actionsMenuRef = useRef<HTMLDivElement | null>(null);
 
@@ -317,10 +322,22 @@ export const DocumentEditor: React.FC<DocumentEditorProps> = ({
   }, [loading, autoFocusTitle, doc?.id]);
 
   useEffect(() => {
-    if (zoomedBlockId && !blocks.some((b) => b.id === zoomedBlockId)) {
+    if (!loading && zoomedBlockId && !blocks.some((b) => b.id === zoomedBlockId)) {
       setZoomedBlockId(null);
     }
-  }, [blocks, zoomedBlockId]);
+  }, [blocks, zoomedBlockId, loading]);
+
+  // Zooming in and out are steps of the browser history: back zooms out again (lib/navHistory.ts)
+  useEffect(() => {
+    if (!embedded) reportNav({ zoom: zoomedBlockId });
+  }, [zoomedBlockId, embedded]);
+  useEffect(
+    () =>
+      onNavStep((place) => {
+        if (!embedded && place.docId === docId) setZoomedBlockId(place.zoom ?? null);
+      }),
+    [embedded, docId]
+  );
 
   const { zoomedBlock, childBlocks, levelOffset } = useMemo(() => {
     return getZoomedSubtree(blocks, zoomedBlockId);