fix: Library page — hover effects, workspace filter, tree, rename

1. Hover effects fixed: converted table to div-based flexbox layout
   (tr/td don't support display:flex). .lib-row now properly shows
   drag handle, checkbox, and OPEN button on hover.

2. Recents filter by workspace: /api/library/recents now accepts
   workspace_id parameter. Template passes active_workspace_id from
   sidebar context to API calls.

3. Tree hierarchy: API now enriches items with has_children via
   _enrich_children() batch query — shows expand chevrons for
   pages with sub-pages.

4. Rename on click: single click on title starts inline rename
   (was opening page). OPEN button still opens side peek.

5. Code cleanup: extracted _enrich_children() helper to eliminate
   duplicate code across 6 endpoints.

app/routers/dashboard.py

@@ -220,6 +220,7 @@ async def library_page(request: Request, owner: str = Query(default=""), repo: s
         sidebar["nav_workspace_id"] = ws["id"] if ws else 0
 
     template = env.get_template("library.html")
+    sidebar["active_workspace_id"] = sidebar.get("nav_workspace_id", 0)
     return template.render(**sidebar)