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/index.css

@@ -76,6 +76,11 @@
   grid-template-rows: 56px 1fr;
   height: 100vh;
   overflow: hidden;
+  transition: grid-template-columns 200ms ease;
+}
+
+.app-shell--collapsed {
+  grid-template-columns: 56px 1fr;
 }
 
 .topbar {