style(page-tree): shrink default tree indentation 16px -> 8px

Port the compact page-tree indentation tweak from the docmost-app
WebKit wrapper (previously applied via injected CSS) into the source.

- doc-tree.tsx: change the default `indentPerLevel` prop from 16 to 8,
  giving an 8px step per nesting level for a more compact page tree.

No callers override the value, so the single default change applies
everywhere DocTree is rendered. EE-hiding injections from docmost-app
were skipped: those surfaces are already removed in this community fork.

apps/client/src/features/page/tree/components/doc-tree.tsx

@@ -121,7 +121,7 @@ function DocTreeInner<T extends object>(
     openIds,
     selectedId,
     renderRow,
-    indentPerLevel = 16,
+    indentPerLevel = 8,
     rowHeight = 32,
     onMove,
     onToggle,