feat(page-tree): gate compact tree density behind COMPACT_PAGE_TREE flag

Make the denser page-tree layout opt-in instead of hardcoded, so row
density can be toggled per deployment via the COMPACT_PAGE_TREE runtime
config flag.

- doc-tree: extract ROW_HEIGHT_STANDARD (32) / ROW_HEIGHT_COMPACT (26);
  default the virtualizer row stride to STANDARD density.
- client: isCompactPageTreeEnabled() in lib/config (reads
  COMPACT_PAGE_TREE, default true); used by space-tree and shared-tree
  to choose the row height.
- server: EnvironmentService.isCompactPageTreeEnabled() and expose
  COMPACT_PAGE_TREE through the window runtime config (static.module).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/client/src/features/share/components/shared-tree.tsx

@@ -25,7 +25,10 @@ import {
   DocTree,
   type DocTreeApi,
   type RenderRowProps,
+  ROW_HEIGHT_COMPACT,
+  ROW_HEIGHT_STANDARD,
 } from "@/features/page/tree/components/doc-tree";
+import { isCompactPageTreeEnabled } from "@/lib/config.ts";
 import { openSharedTreeNodesAtom } from "@/features/share/atoms/open-shared-tree-nodes-atom";
 
 interface SharedTreeProps {
@@ -36,6 +39,7 @@ export default function SharedTree({ sharedPageTree }: SharedTreeProps) {
   const { t } = useTranslation();
   const treeRef = useRef<DocTreeApi | null>(null);
   const { pageSlug } = useParams();
+  const compactTree = isCompactPageTreeEnabled();
   const [openTreeNodes, setOpenTreeNodes] = useAtom(openSharedTreeNodesAtom);
 
   const currentNodeId = extractPageSlugId(pageSlug);
@@ -100,6 +104,7 @@ export default function SharedTree({ sharedPageTree }: SharedTreeProps) {
         renderRow={SharedTreeRow}
         onMove={noopMove}
         onToggle={handleToggle}
+        rowHeight={compactTree ? ROW_HEIGHT_COMPACT : ROW_HEIGHT_STANDARD}
         getDragLabel={getDragLabel}
         aria-label={t("Pages")}
       />