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/split-container.tsx

@@ -33,6 +33,7 @@ import { ResizeHandle } from "@/components/resize-handle";
 import { shouldFocusPaneFromEventTarget } from "@/components/split-container-pane-focus";
 import { usePanelStore } from "@/stores/panel-store";
 import { useWindowControlsPadding } from "@/utils/desktop-window";
+import { TitlebarDragRegion } from "@/components/desktop/titlebar-drag-region";
 import {
   computeTabDropPreview,
   type TabDropPreview,
@@ -887,6 +888,7 @@ function SplitPaneView({
           { paddingLeft: padding.left, paddingRight: padding.right },
         ]}
       >
+        <TitlebarDragRegion />
         <WorkspaceDesktopTabsRow
           paneId={pane.id}
           isFocused={isFocused}
@@ -997,6 +999,7 @@ const styles = StyleSheet.create((theme) => ({
     overflow: "hidden",
   },
   paneTabs: {
+    position: "relative",
     minWidth: 0,
   },
   paneContent: {