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/workspace.py

@@ -0,0 +1,39 @@
+import uuid
+from datetime import datetime
+from typing import Optional
+from pydantic import BaseModel, Field
+
+
+class WorkspaceCreate(BaseModel):
+    name: str = Field(..., min_length=1, max_length=100)
+    slug: str = Field(..., min_length=1, max_length=100, pattern=r'^[a-z0-9-]+$')
+    description: Optional[str] = None
+    icon: Optional[str] = Field(None, max_length=10)
+    accent_color: Optional[str] = Field(None, pattern=r'^#[0-9a-fA-F]{6}$')
+    sort_order: int = 0
+
+
+class WorkspaceUpdate(BaseModel):
+    name: Optional[str] = Field(None, min_length=1, max_length=100)
+    slug: Optional[str] = Field(None, min_length=1, max_length=100, pattern=r'^[a-z0-9-]+$')
+    description: Optional[str] = None
+    icon: Optional[str] = Field(None, max_length=10)
+    accent_color: Optional[str] = Field(None, pattern=r'^#[0-9a-fA-F]{6}$')
+    sort_order: Optional[int] = None
+
+
+class WorkspaceResponse(BaseModel):
+    id: uuid.UUID
+    name: str
+    slug: str
+    description: Optional[str] = None
+    icon: Optional[str] = None
+    accent_color: Optional[str] = None
+    account_id: uuid.UUID
+    is_default: bool
+    sort_order: int
+    tree_count: int = 0
+    created_at: datetime
+    updated_at: datetime
+
+    model_config = {"from_attributes": True}