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/components/layout/NavItem.tsx

@@ -8,9 +8,10 @@ interface NavItemProps {
   label: string
   badge?: number | 'dot'
   matchPaths?: string[]
+  collapsed?: boolean
 }
 
-export function NavItem({ href, icon: Icon, label, badge, matchPaths }: NavItemProps) {
+export function NavItem({ href, icon: Icon, label, badge, matchPaths, collapsed }: NavItemProps) {
   const location = useLocation()
   const isActive = matchPaths
     ? matchPaths.some(p => location.pathname.startsWith(p))
@@ -18,6 +19,31 @@ export function NavItem({ href, icon: Icon, label, badge, matchPaths }: NavItemP
       ? location.pathname === '/'
       : location.pathname.startsWith(href)
 
+  if (collapsed) {
+    return (
+      <Link
+        to={href}
+        className={cn(
+          'group relative flex items-center justify-center rounded-lg p-2 transition-all duration-120',
+          isActive
+            ? 'bg-[hsl(var(--sidebar-active))] text-foreground'
+            : 'text-muted-foreground hover:bg-[hsl(var(--sidebar-hover))] hover:text-foreground'
+        )}
+        title={label}
+      >
+        {isActive && (
+          <div className="absolute left-0 top-1/2 h-6 w-[3px] -translate-y-1/2 rounded-r-full bg-gradient-brand" />
+        )}
+        <Icon size={18} className={cn('shrink-0', isActive ? 'opacity-100' : 'opacity-70')} />
+        {badge !== undefined && badge !== 0 && badge !== 'dot' && (
+          <span className="absolute -right-0.5 -top-0.5 flex h-4 w-4 items-center justify-center rounded-full bg-primary text-[0.5rem] font-bold text-primary-foreground">
+            {badge}
+          </span>
+        )}
+      </Link>
+    )
+  }
+
   return (
     <Link
       to={href}