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/index.css

@@ -27,6 +27,7 @@
     --radius: 0.75rem;
 
     /* App Shell tokens */
+    --sidebar-w: 260px;
     --sidebar-bg: 240 10% 4.5%;
     --sidebar-hover: 240 6% 12%;
     --sidebar-active: 243 75% 59% / 0.08;
@@ -68,6 +69,50 @@
   }
 }
 
+/* App Shell Grid Layout */
+.app-shell {
+  display: grid;
+  grid-template-columns: var(--sidebar-w) 1fr;
+  grid-template-rows: 56px 1fr;
+  height: 100vh;
+  overflow: hidden;
+}
+
+.topbar {
+  grid-column: 1 / -1;
+}
+
+.sidebar {
+  overflow-y: auto;
+}
+
+.main-content {
+  overflow-y: auto;
+}
+
+/* Mobile: single column */
+@media (max-width: 767px) {
+  .app-shell {
+    grid-template-columns: 1fr;
+  }
+}
+
+/* Staggered fade-in for page sections */
+.fade-in {
+  animation: fadeIn 0.3s ease forwards;
+}
+
+@keyframes fadeIn {
+  from { opacity: 0; transform: translateY(6px); }
+  to { opacity: 1; transform: translateY(0); }
+}
+
+/* Workspace switch dropdown */
+@keyframes dropIn {
+  from { opacity: 0; transform: translateY(-4px); }
+  to { opacity: 1; transform: translateY(0); }
+}
+
 /* Animations */
 @keyframes fade-in {
   from { opacity: 0; }