fix(library+workspace): arborescence dans Workspace tab, icônes drag+multi-select visibles, inline rename au clic

- Library: ajout /api/library/children/{id} pour charger les enfants à la demande
- Library: drag handle (.drag-handle) et checkbox (.lib-checkbox) toujours visibles
- Library: SortableJS init pour drag-and-drop réordonner les rows
- Library: toggleExpand() async avec chargement des enfants depuis l'API
- Local workspace: drag handle 6-dots + checkbox toujours visible dans les tree items
- Local workspace: renderChildren() inclut checkbox + drag handle + inline rename
- Local workspace: clic sur nom de fichier = inline rename (plus navigation directe)
- Local workspace: bouton Open pour ouvrir le fichier (double-clic aussi)
- CSS: .item-checkbox toujours visible (plus opacity:0)
- CSS: .drag-handle avec opacité .45 par défaut, 1.0 au hover

app/routers/library.py

@@ -70,6 +70,7 @@ def _build_item(db_row: dict, uid: int = 1) -> dict:
         "id": page_id,
         "title": title,
         "icon": icon,
+        "is_folder": bool(db_row.get("is_folder", 0)),
         "source_type": source_type,
         "source_label": _source_label(source_type, workspace),
         "workspace": workspace,
@@ -254,6 +255,22 @@ async def library_private(
     return {"items": items}
 
 
+@router.get("/children/{page_id:int}")
+async def library_children(page_id: int, request: Request):
+    """Return child pages for a given parent page (for tree expansion in Library)."""
+    uid = _get_user_id(request)
+    with get_conn() as conn:
+        rows = conn.execute(
+            f"{BASE_SELECT}, p.is_folder, (SELECT COUNT(*) FROM pages c WHERE c.parent_id = p.id AND c.deleted_at IS NULL) as child_count "
+            f"FROM pages p WHERE p.parent_id = ? AND p.deleted_at IS NULL "
+            f"ORDER BY p.is_folder DESC, p.title COLLATE NOCASE",
+            [page_id],
+        ).fetchall()
+    items = _rows_to_items(rows, uid)
+    _enrich_children(items)
+    return {"items": items}
+
+
 @router.get("/workspace")
 async def library_workspace(
     request: Request,