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/agent/draft-agent-screen.tsx

@@ -51,7 +51,7 @@ import type {
 } from "@server/server/agent/agent-sdk-types";
 import { AGENT_PROVIDER_DEFINITIONS } from "@server/server/agent/provider-manifest";
 import { prepareWorkspaceTab } from "@/utils/workspace-navigation";
-import { TitlebarDragRegion, TitlebarNoDragContent } from "@/components/desktop/titlebar-drag-region";
+import { TitlebarDragRegion } from "@/components/desktop/titlebar-drag-region";
 import { useKeyboardShiftStyle } from "@/hooks/use-keyboard-shift-style";
 import { normalizeAgentSnapshot } from "@/utils/agent-snapshots";
 import { useAgentInputDraft } from "@/hooks/use-agent-input-draft";
@@ -1000,8 +1000,7 @@ function DraftAgentScreenContent({
   const mainContent = (
     <View style={styles.container}>
       <TitlebarDragRegion />
-      <TitlebarNoDragContent direction="column">
-        <View style={styles.outerContainer}>
+      <View style={styles.outerContainer}>
         <View style={styles.agentPanel}>
           <View
             style={[
@@ -1264,7 +1263,6 @@ function DraftAgentScreenContent({
           />
         ) : null}
       </View>
-      </TitlebarNoDragContent>
     </View>
   );