feat: design system overhaul — sidebar, AI chats, settings, brainstorm, color cleanup

- Sidebar: dynamic brand-accent colors, brainstorm section restyled
- AI chat general: popup panel with expand/collapse, hides when contextual AI open
- AI chat contextual: tabs reordered (Actions first), X close button, height fix
- Settings: all tabs restyled, 6 new color presets (sage, terracotta, iron, etc.)
- Global color cleanup: emerald/orange hardcoded → brand-accent dynamic
- Brainstorm page: orange → brand-accent throughout
- PageEntry animation component added to key pages
- Floating AI button: bg-brand-accent instead of hardcoded black
- i18n: all 15 locales updated with new AI/billing keys
- Billing: freemium quota tracking, BYOK, stripe subscription scaffolding
- Admin: integrated into new design
- AGENTS.md + CLAUDE.md project rules added

memento-note/app/api/notebooks/[id]/route.ts

@@ -28,7 +28,7 @@ export async function PATCH(
   try {
     const { id } = await params
     const body = await request.json()
-    const { name, icon, color, order, trashedAt } = body
+    const { name, icon, color, order, trashedAt, parentId } = body
 
     const existing = await prisma.notebook.findUnique({
       where: { id },
@@ -49,12 +49,40 @@ export async function PATCH(
       )
     }
 
+    if (parentId !== undefined) {
+      if (parentId !== null) {
+        if (parentId === id) {
+          return NextResponse.json(
+            { success: false, error: 'A notebook cannot be its own parent' },
+            { status: 400 }
+          )
+        }
+        const target = await prisma.notebook.findFirst({
+          where: { id: parentId, userId: session.user.id }
+        })
+        if (!target) {
+          return NextResponse.json(
+            { success: false, error: 'Parent notebook not found' },
+            { status: 404 }
+          )
+        }
+        const descendants = await getDescendantIds(id)
+        if (descendants.includes(parentId)) {
+          return NextResponse.json(
+            { success: false, error: 'Cannot move a notebook into its own descendant' },
+            { status: 400 }
+          )
+        }
+      }
+    }
+
     const updateData: any = {}
     if (name !== undefined) updateData.name = name.trim()
     if (icon !== undefined) updateData.icon = icon
     if (color !== undefined) updateData.color = color
     if (order !== undefined) updateData.order = order
     if (trashedAt !== undefined) updateData.trashedAt = trashedAt
+    if (parentId !== undefined) updateData.parentId = parentId
 
     if (trashedAt !== undefined) {
       const descendantIds = await getDescendantIds(id)