fix: rewrite titlebar drag to match VS Code's static pattern

Replace the stateful TitlebarDragRegion (hooks, ResizeObserver, IPC,
fullscreen tracking) with a pure static component — matching VS Code's
titlebar-drag-region exactly: position absolute, full size, no z-index,
no pointer-events, no state, no event listeners.

Remove TitlebarNoDragContent entirely — VS Code doesn't wrap content in
no-drag; interactive elements get no-drag from the global CSS backstop
in index.html.

Add drag regions to all header surfaces:
- ScreenHeader (sessions, workspace header)
- Left sidebar (traffic light area + header)
- Split container pane tabs
- Explorer sidebar header (Changes/Files tabs)

Fix workspace header empty space not draggable by changing
headerTitleContainer from flex: 1 to flexShrink: 1.

packages/app/src/components/explorer-sidebar.tsx

@@ -18,6 +18,7 @@ import { GitDiffPane } from "./git-diff-pane";
 import { FileExplorerPane } from "./file-explorer-pane";
 import { useKeyboardShiftStyle } from "@/hooks/use-keyboard-shift-style";
 import { useWindowControlsPadding } from "@/utils/desktop-window";
+import { TitlebarDragRegion } from "@/components/desktop/titlebar-drag-region";
 
 const MIN_CHAT_WIDTH = 400;
 function logExplorerSidebar(_event: string, _details: Record<string, unknown>): void {}
@@ -344,6 +345,7 @@ function SidebarContent({
     <View style={styles.sidebarContent} pointerEvents="auto">
       {/* Header with tabs and close button */}
       <View style={[styles.header, { paddingRight: padding.right }]} testID="explorer-header">
+        <TitlebarDragRegion />
         <View style={styles.tabsContainer}>
           {isGit && (
             <Pressable
@@ -434,6 +436,7 @@ const styles = StyleSheet.create((theme) => ({
     overflow: "hidden",
   },
   header: {
+    position: "relative",
     height: HEADER_INNER_HEIGHT,
     flexDirection: "row",
     alignItems: "center",