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/api/endpoints/workspaces.py

@@ -0,0 +1,154 @@
+from typing import Annotated
+from uuid import UUID
+from fastapi import APIRouter, Depends, HTTPException, status
+from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy import select, func
+
+from app.core.database import get_db
+from app.models.workspace import Workspace
+from app.models.tree import Tree
+from app.models.user import User
+from app.schemas.workspace import WorkspaceCreate, WorkspaceUpdate, WorkspaceResponse
+from app.api.deps import get_current_active_user
+
+router = APIRouter(prefix="/workspaces", tags=["workspaces"])
+
+
+@router.get("", response_model=list[WorkspaceResponse])
+async def list_workspaces(
+    db: Annotated[AsyncSession, Depends(get_db)],
+    current_user: Annotated[User, Depends(get_current_active_user)],
+):
+    """List all workspaces for the user's account."""
+    if not current_user.account_id:
+        return []
+
+    # Get workspaces with tree counts
+    query = (
+        select(
+            Workspace,
+            func.count(Tree.id).label("tree_count")
+        )
+        .outerjoin(Tree, (Tree.workspace_id == Workspace.id) & (Tree.deleted_at.is_(None)))
+        .where(Workspace.account_id == current_user.account_id)
+        .group_by(Workspace.id)
+        .order_by(Workspace.sort_order, Workspace.name)
+    )
+    result = await db.execute(query)
+    rows = result.all()
+
+    return [
+        WorkspaceResponse(
+            **{c.key: getattr(ws, c.key) for c in Workspace.__table__.columns},
+            tree_count=tree_count
+        )
+        for ws, tree_count in rows
+    ]
+
+
+@router.post("", response_model=WorkspaceResponse, status_code=status.HTTP_201_CREATED)
+async def create_workspace(
+    data: WorkspaceCreate,
+    db: Annotated[AsyncSession, Depends(get_db)],
+    current_user: Annotated[User, Depends(get_current_active_user)],
+):
+    """Create a new workspace."""
+    if not current_user.account_id:
+        raise HTTPException(status_code=400, detail="No account found")
+
+    # Check slug uniqueness within account
+    existing = await db.execute(
+        select(Workspace).where(
+            Workspace.account_id == current_user.account_id,
+            Workspace.slug == data.slug
+        )
+    )
+    if existing.scalar_one_or_none():
+        raise HTTPException(status_code=409, detail="Workspace slug already exists")
+
+    workspace = Workspace(
+        name=data.name,
+        slug=data.slug,
+        description=data.description,
+        icon=data.icon,
+        accent_color=data.accent_color,
+        account_id=current_user.account_id,
+        sort_order=data.sort_order,
+    )
+    db.add(workspace)
+    await db.flush()
+    await db.commit()
+
+    return WorkspaceResponse(
+        **{c.key: getattr(workspace, c.key) for c in Workspace.__table__.columns},
+        tree_count=0
+    )
+
+
+@router.patch("/{workspace_id}", response_model=WorkspaceResponse)
+async def update_workspace(
+    workspace_id: UUID,
+    data: WorkspaceUpdate,
+    db: Annotated[AsyncSession, Depends(get_db)],
+    current_user: Annotated[User, Depends(get_current_active_user)],
+):
+    """Update a workspace."""
+    workspace = await db.get(Workspace, workspace_id)
+    if not workspace or workspace.account_id != current_user.account_id:
+        raise HTTPException(status_code=404, detail="Workspace not found")
+
+    update_data = data.model_dump(exclude_unset=True)
+    if "slug" in update_data:
+        existing = await db.execute(
+            select(Workspace).where(
+                Workspace.account_id == current_user.account_id,
+                Workspace.slug == update_data["slug"],
+                Workspace.id != workspace_id
+            )
+        )
+        if existing.scalar_one_or_none():
+            raise HTTPException(status_code=409, detail="Workspace slug already exists")
+
+    for key, value in update_data.items():
+        setattr(workspace, key, value)
+
+    await db.commit()
+
+    # Get tree count
+    count_result = await db.execute(
+        select(func.count(Tree.id)).where(
+            Tree.workspace_id == workspace_id,
+            Tree.deleted_at.is_(None)
+        )
+    )
+    tree_count = count_result.scalar() or 0
+
+    return WorkspaceResponse(
+        **{c.key: getattr(workspace, c.key) for c in Workspace.__table__.columns},
+        tree_count=tree_count
+    )
+
+
+@router.delete("/{workspace_id}", status_code=status.HTTP_204_NO_CONTENT)
+async def delete_workspace(
+    workspace_id: UUID,
+    db: Annotated[AsyncSession, Depends(get_db)],
+    current_user: Annotated[User, Depends(get_current_active_user)],
+):
+    """Delete a workspace. Trees are unassigned, not deleted."""
+    workspace = await db.get(Workspace, workspace_id)
+    if not workspace or workspace.account_id != current_user.account_id:
+        raise HTTPException(status_code=404, detail="Workspace not found")
+
+    if workspace.is_default:
+        raise HTTPException(status_code=400, detail="Cannot delete the default workspace")
+
+    # Unassign trees (set workspace_id to NULL)
+    await db.execute(
+        Tree.__table__.update()
+        .where(Tree.workspace_id == workspace_id)
+        .values(workspace_id=None)
+    )
+
+    await db.delete(workspace)
+    await db.commit()