feat: hierarchical notebook system - trash, selectors, breadcrumb, sidebar tree

- Schema: soft delete with trashedAt on Notebook model
- API: PATCH/GET notebooks support trashedAt filtering with cascade
- Sidebar: recursive tree rendering with collapse/expand, visual guides, hover actions
- HierarchicalNotebookSelector: portal-based dropdown with search, breadcrumbs, dropUp support
- AI chat: context selector with Toutes mes notes + notebook selector
- Agent detail: flat selects replaced with HierarchicalNotebookSelector
- Breadcrumb: notebook path display on home page
- Trash view: card grid with countdown, restore/permanent delete
- CSS: design tokens (ink, paper, blueprint, concrete, etc.)
- Types: parentId, trashedAt added to Notebook interface

memento-note/prisma/schema.prisma

@@ -83,6 +83,7 @@ model Notebook {
   color         String?
   order         Int
   parentId      String?
+  trashedAt     DateTime?
   userId        String
   createdAt     DateTime       @default(now())
   updatedAt     DateTime       @updatedAt
@@ -98,6 +99,7 @@ model Notebook {
   @@index([userId, order])
   @@index([userId])
   @@index([parentId])
+  @@index([trashedAt])
 }
 
 model Label {