ui: tidy sidebar tree indent and consolidate sidebar toggles into filter bar

Folder tree now indents 20px per level (chevron width + gap) so a child's
chevron column lines up under its parent's label, and depth-1 rows nest
under the section eyebrow instead of starting flush with it. Spacer for
leaf rows matches the chevron button footprint so rows align regardless
of expandability.

Sidebar open/close buttons (previously split between TopBar and each
panel header) collapse into two toggles at the ends of the FilterBar.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/layout/LeftSidebar.tsx

@@ -15,7 +15,6 @@ import {
   Layers2,
   MoreHorizontal,
   Pencil,
-  PanelLeftClose,
   Settings,
   Users,
   Eye,
@@ -64,11 +63,7 @@ interface TreeItem {
   isHidden?: boolean
 }
 
-interface LeftSidebarProps {
-  onCollapse: () => void
-}
-
-export function LeftSidebar({ onCollapse }: LeftSidebarProps) {
+export function LeftSidebar() {
   const { user, isAdmin, logout } = useAuth()
   const [expandedItems, setExpandedItems] = useState<Set<string>>(new Set(['library', 'folders', 'heaps']))
   // Inline rename state for source-root rows. Stores the id being edited
@@ -839,14 +834,6 @@ export function LeftSidebar({ onCollapse }: LeftSidebarProps) {
           >
             <UploadIcon className="h-3.5 w-3.5" />
           </button>
-          <button
-            onClick={onCollapse}
-            className="rounded p-0.5 text-text-muted hover:bg-surface-2 hover:text-text"
-            title="Collapse panel (Tab)"
-            aria-label="Collapse panel"
-          >
-            <PanelLeftClose className="h-3.5 w-3.5" />
-          </button>
         </div>
       </div>
       {/* Active heap card — pinned just below the Library header so