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/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: