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/App.tsx

@@ -56,12 +56,7 @@ function MainApp() {
 
   return (
     <div className="flex flex-col h-screen bg-bg text-text">
-      <TopBar
-        leftSidebarOpen={leftSidebarOpen}
-        rightSidebarOpen={showRightSidebar}
-        onExpandLeft={() => setLeftSidebarOpen(true)}
-        onExpandRight={() => setRightSidebarOpen(true)}
-      />
+      <TopBar />
 
       <div className="flex flex-1 overflow-hidden">
         {/* Left Sidebar */}
@@ -70,9 +65,7 @@ function MainApp() {
             leftSidebarOpen ? 'w-60' : 'w-0'
           } overflow-hidden border-r border-border bg-surface`}
         >
-          <LeftSidebar
-            onCollapse={() => setLeftSidebarOpen(false)}
-          />
+          <LeftSidebar />
         </div>
 
         {/* Main column — filter bar, discard bar, timeline. Lives to the
@@ -80,7 +73,14 @@ function MainApp() {
          *  across the sidebar. relative so the KeyboardHints overlay
          *  centers against this column, not the viewport. */}
         <div className="relative flex min-w-0 flex-1 flex-col">
-          {!isSettings && <FilterBar />}
+          {!isSettings && (
+            <FilterBar
+              leftSidebarOpen={leftSidebarOpen}
+              rightSidebarOpen={showRightSidebar}
+              onToggleLeftSidebar={() => setLeftSidebarOpen(!leftSidebarOpen)}
+              onToggleRightSidebar={() => setRightSidebarOpen(!rightSidebarOpen)}
+            />
+          )}
           {!isSettings && <DiscardActionBar />}
           <div className="flex-1 overflow-auto">
             {currentSection === 'settings' ? (
@@ -110,7 +110,7 @@ function MainApp() {
             showRightSidebar ? 'w-72' : 'w-0'
           } overflow-hidden border-l border-border bg-surface`}
         >
-          <RightSidebar onCollapse={() => setRightSidebarOpen(false)} />
+          <RightSidebar />
         </div>
       </div>