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/models/tree.py

@@ -15,6 +15,7 @@ if TYPE_CHECKING:
     from app.models.tag import TreeTag
     from app.models.folder import UserFolder
     from app.models.tree_share import TreeShare
+    from app.models.workspace import Workspace
 
 
 class Tree(Base):
@@ -120,6 +121,13 @@ class Tree(Base):
         onupdate=lambda: datetime.now(timezone.utc)
     )
     usage_count: Mapped[int] = mapped_column(Integer, default=0)
+    workspace_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("workspaces.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+        comment="Workspace this tree belongs to (organizational context)"
+    )
 
     # Fork tracking
     parent_tree_id: Mapped[Optional[uuid.UUID]] = mapped_column(
@@ -184,6 +192,8 @@ class Tree(Base):
         cascade="all, delete-orphan"
     )
 
+    workspace: Mapped[Optional["Workspace"]] = relationship("Workspace", back_populates="trees")
+
     # New organization relationships
     category_rel: Mapped[Optional["TreeCategory"]] = relationship("TreeCategory", back_populates="trees")
     tags: Mapped[list["TreeTag"]] = relationship(