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/open-project-screen.tsx

@@ -9,7 +9,7 @@ import { useOpenProjectPicker } from "@/hooks/use-open-project-picker";
 import { usePanelStore } from "@/stores/panel-store";
 import { useSessionStore } from "@/stores/session-store";
 import { isCompactFormFactor } from "@/constants/layout";
-import { TitlebarDragRegion, TitlebarNoDragContent } from "@/components/desktop/titlebar-drag-region";
+import { TitlebarDragRegion } from "@/components/desktop/titlebar-drag-region";
 
 export function OpenProjectScreen({ serverId }: { serverId: string }) {
   const openAgentList = usePanelStore((s) => s.openAgentList);
@@ -30,24 +30,22 @@ export function OpenProjectScreen({ serverId }: { serverId: string }) {
       <MenuHeader borderless />
       <View style={styles.content}>
         <TitlebarDragRegion />
-        <TitlebarNoDragContent direction="column">
-          <View style={styles.logo}>
-            <PaseoLogo size={56} />
-          </View>
-          <View style={styles.headingGroup}>
-            <Text style={styles.heading}>What shall we build today?</Text>
-            {hasHydrated && !hasProjects ? (
-              <Text style={styles.subtitle}>
-                Add a project folder to start running agents on your codebase
-              </Text>
-            ) : null}
-          </View>
-          <View style={styles.cta}>
-            <Button variant="default" leftIcon={FolderOpen} onPress={() => void openProjectPicker()} testID="open-project-submit">
-              Add a project
-            </Button>
-          </View>
-        </TitlebarNoDragContent>
+        <View style={styles.logo}>
+          <PaseoLogo size={56} />
+        </View>
+        <View style={styles.headingGroup}>
+          <Text style={styles.heading}>What shall we build today?</Text>
+          {hasHydrated && !hasProjects ? (
+            <Text style={styles.subtitle}>
+              Add a project folder to start running agents on your codebase
+            </Text>
+          ) : null}
+        </View>
+        <View style={styles.cta}>
+          <Button variant="default" leftIcon={FolderOpen} onPress={() => void openProjectPicker()} testID="open-project-submit">
+            Add a project
+          </Button>
+        </View>
       </View>
     </View>
   );