fix(desktop): restore sidebar titlebar dragging

The sidebar title area also needs to participate in Electron's CSS app-region chrome. Mark only the sidebar title/header region as draggable and keep the full sidebar body out of the drag region so session rows, search controls, and project actions remain normal interactions.

Constraint: Electron window dragging is driven by CSS app-region regions after the migration
Rejected: Keep the sidebar-wide startDragging mouse handler | Electron does not use that as the active drag path and it misses the left title gap
Confidence: high
Scope-risk: narrow
Directive: Keep sidebar body interactions outside the drag region unless a real titlebar affordance is added there
Tested: cd desktop && bun run test src/components/layout/Sidebar.test.tsx --run
Tested: bun run check:desktop

desktop/src/components/layout/Sidebar.test.tsx

@@ -255,6 +255,7 @@ describe('Sidebar', () => {
     ])
     expect(useTabStore.getState().activeTabId).toBe('session-new-1')
     expect(screen.getByRole('complementary')).not.toHaveAttribute('data-desktop-drag-region')
+    expect(screen.getByTestId('sidebar-title-region')).toHaveAttribute('data-desktop-drag-region')
   })
 
   it('groups sessions by project and expands overflow rows', () => {

desktop/src/components/layout/Sidebar.tsx

@@ -15,7 +15,6 @@ import { publicAssetPath } from '../../lib/publicAsset'
 const desktopHost = getDesktopHost()
 const isDesktopRuntime = desktopHost.isDesktop
 const canUseNativeDialogs = desktopHost.capabilities.dialogs
-const canStartWindowDragging = desktopHost.capabilities.windowControls
 const isWindows = typeof navigator !== 'undefined' && /Win/.test(navigator.platform)
 const SESSION_LIST_AUTO_REFRESH_MS = 30_000
 const SESSION_LIST_FOCUS_REFRESH_MIN_MS = 5_000
@@ -575,19 +574,6 @@ export function Sidebar({ isMobile = false, onRequestClose }: SidebarProps) {
     setRenameValue('')
   }, [renamingId, renameValue, renameSession])
 
-  const startDraggingRef = useRef<(() => Promise<void>) | null>(null)
-
-  useEffect(() => {
-    if (!canStartWindowDragging) return
-    startDraggingRef.current = () => getDesktopHost().window.startDragging()
-  }, [])
-
-  const handleSidebarDrag = useCallback((e: React.MouseEvent) => {
-    if (e.button !== 0) return
-    if ((e.target as HTMLElement).closest('button, input, textarea, select, a, [role="button"]')) return
-    startDraggingRef.current?.()
-  }, [])
-
   useEffect(() => {
     if (!isBatchMode) return
 
@@ -611,12 +597,15 @@ export function Sidebar({ isMobile = false, onRequestClose }: SidebarProps) {
 
   return (
     <aside
-      onMouseDown={handleSidebarDrag}
       className="sidebar-panel relative h-full flex flex-col bg-[var(--color-surface-sidebar)] border-r border-[var(--color-border)] select-none"
       data-state={expanded ? 'open' : 'closed'}
       aria-label="Sidebar"
     >
-      <div className={`px-3 pb-2 ${isDesktopRuntime && !isWindows ? 'pt-[44px]' : 'pt-3'}`}>
+      <div
+        data-testid="sidebar-title-region"
+        data-desktop-drag-region
+        className={`px-3 pb-2 ${isDesktopRuntime && !isWindows ? 'pt-[44px]' : 'pt-3'}`}
+      >
         <div className={`flex ${expanded ? 'items-center justify-between gap-3' : 'flex-col items-center gap-2'}`}>
           <div className={`flex min-w-0 items-center ${expanded ? 'gap-2.5' : 'justify-center'}`}>
             <img src={publicAssetPath('app-icon.png')} alt="" className="h-8 w-8 flex-shrink-0" />