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/workspace.py

@@ -0,0 +1,57 @@
+import uuid
+from datetime import datetime, timezone
+from typing import Optional, TYPE_CHECKING
+from sqlalchemy import String, Text, DateTime, ForeignKey, Boolean, Integer, UniqueConstraint
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy.dialects.postgresql import UUID
+from app.core.database import Base
+
+if TYPE_CHECKING:
+    from app.models.account import Account
+    from app.models.tree import Tree
+    from app.models.category import TreeCategory
+
+
+class Workspace(Base):
+    """Workspaces are the top-level organizational context for trees/flows.
+
+    They sit above the folder system — a workspace scopes which trees/flows
+    are visible, while folders remain for personal organization within.
+    """
+    __tablename__ = "workspaces"
+    __table_args__ = (
+        UniqueConstraint('slug', 'account_id', name='uq_workspaces_slug_account'),
+    )
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        primary_key=True,
+        default=uuid.uuid4
+    )
+    name: Mapped[str] = mapped_column(String(100), nullable=False)
+    slug: Mapped[str] = mapped_column(String(100), nullable=False, index=True)
+    description: Mapped[Optional[str]] = mapped_column(Text, nullable=True)
+    icon: Mapped[Optional[str]] = mapped_column(String(10), nullable=True)
+    accent_color: Mapped[Optional[str]] = mapped_column(String(7), nullable=True)
+    account_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("accounts.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True
+    )
+    is_default: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
+    sort_order: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc)
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc),
+        onupdate=lambda: datetime.now(timezone.utc)
+    )
+
+    # Relationships
+    account: Mapped["Account"] = relationship("Account", back_populates="workspaces")
+    trees: Mapped[list["Tree"]] = relationship("Tree", back_populates="workspace")
+    categories: Mapped[list["TreeCategory"]] = relationship("TreeCategory", back_populates="workspace")