feat: add sidebar collapse, category/tag filtering, and workspace CRUD (Phase D)

- Sidebar collapse/expand toggle with icon-only rail mode (persisted)
- Sidebar category/tag clicks navigate to /trees with URL params
- TreeLibraryPage syncs filters from URL search params bidirectionally
- Workspace create modal with icon picker and auto-slug generation
- TopBar logo adapts to collapsed sidebar state

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/store/workspaceStore.ts

@@ -6,15 +6,18 @@ interface WorkspaceState {
   workspaces: Workspace[]
   activeWorkspaceId: string | null
   loading: boolean
+  sidebarCollapsed: boolean
   setActiveWorkspace: (id: string) => void
   fetchWorkspaces: () => Promise<void>
   getActiveWorkspace: () => Workspace | undefined
+  toggleSidebar: () => void
 }
 
 export const useWorkspaceStore = create<WorkspaceState>((set, get) => ({
   workspaces: [],
   activeWorkspaceId: localStorage.getItem('active-workspace-id'),
   loading: false,
+  sidebarCollapsed: localStorage.getItem('sidebar-collapsed') === 'true',
 
   setActiveWorkspace: (id: string) => {
     localStorage.setItem('active-workspace-id', id)
@@ -47,4 +50,10 @@ export const useWorkspaceStore = create<WorkspaceState>((set, get) => ({
     const { workspaces, activeWorkspaceId } = get()
     return workspaces.find(w => w.id === activeWorkspaceId)
   },
+
+  toggleSidebar: () => {
+    const next = !get().sidebarCollapsed
+    localStorage.setItem('sidebar-collapsed', String(next))
+    set({ sidebarCollapsed: next })
+  },
 }))