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/RightSidebar.tsx

@@ -1,5 +1,5 @@
 import { useState } from 'react'
-import { X, Star, ShoppingBasket, Trash2, Plus, PanelRightClose } from 'lucide-react'
+import { X, Star, ShoppingBasket, Trash2, Plus } from 'lucide-react'
 import clsx from 'clsx'
 import { useMutation, useQueryClient } from '@tanstack/react-query'
 import { format } from 'date-fns'
@@ -29,11 +29,7 @@ import { COLOR_LABEL_OPTIONS } from '../../constants/colorLabels'
  *  - 2+ photos selected → renders a slim bulk-action panel that fans out
  *    rating / color / discard / pick across the entire selection.
  */
-interface RightSidebarProps {
-  onCollapse: () => void
-}
-
-export function RightSidebar({ onCollapse }: RightSidebarProps) {
+export function RightSidebar() {
   const { selectedPhotos, activePhotoId, clearSelection } = usePhotoStore()
   const queryClient = useQueryClient()
 
@@ -221,14 +217,6 @@ export function RightSidebar({ onCollapse }: RightSidebarProps) {
             <X 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 (I)"
-          aria-label="Collapse panel"
-        >
-          <PanelRightClose className="h-3.5 w-3.5" />
-        </button>
       </div>
     </div>
   )