feat: add workspace system and sidebar layout (UI design system Phase A+B)

Backend: Workspace model, migration (036), schemas, CRUD API endpoints.
Adds workspace_id to trees and categories, seeds 4 default workspaces
per account, auto-assigns existing trees by tree_type.

Frontend: Complete AppLayout rewrite from top-nav to CSS Grid shell
with persistent sidebar + topbar. New components: WorkspaceSwitcher,
NavItem, CategoryList, TagCloud, TopBar, Sidebar. Dashboard components:
QuickStats, FiltersBar, SectionGroup, TreeListItem, SessionsPanel.
WorkspaceStore with localStorage persistence.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/schemas/category.py

@@ -36,6 +36,8 @@ class CategoryResponse(CategoryBase):
     account_id: Optional[UUID] = None
     display_order: int
     is_active: bool
+    color: Optional[str] = None
+    workspace_id: Optional[UUID] = None
     created_at: datetime
     updated_at: datetime
     tree_count: int = 0  # Computed field
@@ -52,6 +54,8 @@ class CategoryListResponse(BaseModel):
     account_id: Optional[UUID] = None
     display_order: int
     is_active: bool
+    color: Optional[str] = None
+    workspace_id: Optional[UUID] = None
     tree_count: int = 0
 
     class Config: