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/headers/screen-header.tsx

@@ -9,7 +9,7 @@ import {
   isCompactFormFactor,
 } from "@/constants/layout";
 import { useWindowControlsPadding } from "@/utils/desktop-window";
-import { TitlebarDragRegion, TitlebarNoDragContent } from "@/components/desktop/titlebar-drag-region";
+import { TitlebarDragRegion } from "@/components/desktop/titlebar-drag-region";
 
 interface ScreenHeaderProps {
   left?: ReactNode;
@@ -46,10 +46,8 @@ export function ScreenHeader({ left, right, leftStyle, rightStyle, borderless }:
           ]}
         >
           <TitlebarDragRegion />
-          <TitlebarNoDragContent>
-            <View style={[styles.left, leftStyle]}>{left}</View>
-            <View style={[styles.right, rightStyle]}>{right}</View>
-          </TitlebarNoDragContent>
+          <View style={[styles.left, leftStyle]}>{left}</View>
+          <View style={[styles.right, rightStyle]}>{right}</View>
         </View>
       </View>
     </View>