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/__init__.py

@@ -20,6 +20,7 @@ from .session_share import SessionShare, SessionShareView
 from .account_limit_override import AccountLimitOverride
 from .feature_flag import FeatureFlag, PlanFeatureDefault, AccountFeatureOverride
 from .platform_setting import PlatformSetting
+from .workspace import Workspace
 
 __all__ = [
     "User",
@@ -51,4 +52,5 @@ __all__ = [
     "PlanFeatureDefault",
     "AccountFeatureOverride",
     "PlatformSetting",
+    "Workspace",
 ]

backend/app/models/account.py

@@ -15,6 +15,7 @@ if TYPE_CHECKING:
     from app.models.step_category import StepCategory
     from app.models.step_library import StepLibrary
     from app.models.account_limit_override import AccountLimitOverride
+    from app.models.workspace import Workspace
 
 
 class Account(Base):
@@ -45,3 +46,4 @@ class Account(Base):
     step_categories: Mapped[list["StepCategory"]] = relationship("StepCategory", foreign_keys="[StepCategory.account_id]", back_populates="account")
     step_library: Mapped[list["StepLibrary"]] = relationship("StepLibrary", foreign_keys="[StepLibrary.account_id]", back_populates="account")
     limit_override: Mapped[Optional["AccountLimitOverride"]] = relationship("AccountLimitOverride", back_populates="account", uselist=False)
+    workspaces: Mapped[list["Workspace"]] = relationship("Workspace", back_populates="account")

backend/app/models/category.py

@@ -11,6 +11,7 @@ if TYPE_CHECKING:
     from app.models.team import Team
     from app.models.account import Account
     from app.models.user import User
+    from app.models.workspace import Workspace
 
 
 class TreeCategory(Base):
@@ -47,6 +48,17 @@ class TreeCategory(Base):
     )
     display_order: Mapped[int] = mapped_column(Integer, nullable=False, default=0, index=True)
     is_active: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
+    color: Mapped[Optional[str]] = mapped_column(
+        String(7), nullable=True, default='#3b82f6',
+        comment="Hex color for category dot indicator"
+    )
+    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 category belongs to"
+    )
     created_by: Mapped[Optional[uuid.UUID]] = mapped_column(
         UUID(as_uuid=True),
         ForeignKey("users.id", ondelete="SET NULL"),
@@ -67,6 +79,7 @@ class TreeCategory(Base):
     account: Mapped[Optional["Account"]] = relationship("Account", foreign_keys=[account_id], back_populates="categories")
     creator: Mapped[Optional["User"]] = relationship("User", foreign_keys=[created_by])
     trees: Mapped[list["Tree"]] = relationship("Tree", back_populates="category_rel")
+    workspace: Mapped[Optional["Workspace"]] = relationship("Workspace", back_populates="categories")
 
     @property
     def is_global(self) -> bool:

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(

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")