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/screens/workspace/workspace-screen.tsx

@@ -2327,7 +2327,7 @@ const styles = StyleSheet.create((theme) => ({
     flexShrink: 1,
   },
   headerTitleContainer: {
-    flex: 1,
+    flexShrink: 1,
     minWidth: 0,
     flexDirection: "row",
     alignItems: "center",