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>

frontend/src/store/workspaceStore.ts

@@ -0,0 +1,50 @@
+import { create } from 'zustand'
+import type { Workspace } from '@/types'
+import { workspacesApi } from '@/api/workspaces'
+
+interface WorkspaceState {
+  workspaces: Workspace[]
+  activeWorkspaceId: string | null
+  loading: boolean
+  setActiveWorkspace: (id: string) => void
+  fetchWorkspaces: () => Promise<void>
+  getActiveWorkspace: () => Workspace | undefined
+}
+
+export const useWorkspaceStore = create<WorkspaceState>((set, get) => ({
+  workspaces: [],
+  activeWorkspaceId: localStorage.getItem('active-workspace-id'),
+  loading: false,
+
+  setActiveWorkspace: (id: string) => {
+    localStorage.setItem('active-workspace-id', id)
+    set({ activeWorkspaceId: id })
+  },
+
+  fetchWorkspaces: async () => {
+    set({ loading: true })
+    try {
+      const workspaces = await workspacesApi.list()
+      const state = get()
+      let activeId = state.activeWorkspaceId
+
+      // If no active workspace or active workspace doesn't exist, use default
+      if (!activeId || !workspaces.find(w => w.id === activeId)) {
+        const defaultWs = workspaces.find(w => w.is_default) || workspaces[0]
+        if (defaultWs) {
+          activeId = defaultWs.id
+          localStorage.setItem('active-workspace-id', activeId)
+        }
+      }
+
+      set({ workspaces, activeWorkspaceId: activeId, loading: false })
+    } catch {
+      set({ loading: false })
+    }
+  },
+
+  getActiveWorkspace: () => {
+    const { workspaces, activeWorkspaceId } = get()
+    return workspaces.find(w => w.id === activeWorkspaceId)
+  },
+}))