Die oberste Sidebar-Ebene ist jetzt eine gemischte TopLevel-Collection, so
dass eine Liste auch oberhalb eines Ordners stehen kann. DropAsync deckt
alle vier Quelle/Ziel-Kombinationen ab, ReorderSidebarAsync numeriert die
gesamte sichtbare Reihenfolge ueber lists und list_folders hinweg.
Data: folder delete keeps its lists (ON DELETE SET NULL), rename patches
name/icon without resetting sort order or collapse state.
Sidebar: grouping on load, folder badge as the sum of its children, both
move routes, and the persisted collapse state.
Drops the LoadAsync/AddRowAsync normalization of a dangling folder_id:
FK enforcement is on for every connection, so the row it guarded against
cannot exist, and ContainerOf already falls back to the top level.
mika kuns