Restore the "Drop files here" backdrop when dragging files (#1801)

Decouples file drag-and-drop into a FileDropZone provider (drag target + backdrop) and a useFileDrop consumer hook, with drag state on a Reanimated shared value so dragging triggers no React re-renders.

Fixes the collapsed/invisible backdrop (the drop zone had been moved into the composer with flex:0, collapsing the overlay to zero height) and restores whole-surface coverage across the agent panel, New Workspace screen, draft workspace tab, and workspace-setup dialog. The backdrop and copy cursor only appear when a drop will actually be accepted (a consumer is mounted and not submitting).

Refs #520

packages/app/src/screens/new-workspace-screen.tsx

@@ -11,6 +11,7 @@ import { createNameId } from "mnemonic-id";
 import { useQuery } from "@tanstack/react-query";
 import { Check, ChevronDown, Folder, GitBranch, GitPullRequest, X } from "lucide-react-native";
 import { Composer } from "@/composer";
+import { FileDropZone } from "@/components/file-drop/file-drop-zone";
 import { DraftAgentModeControl } from "@/composer/agent-controls/mode-control";
 import { splitComposerAttachmentsForSubmit } from "@/composer/attachments/submit";
 import { HostStatusDot } from "@/components/host-status-dot";
@@ -1967,7 +1968,7 @@ export function NewWorkspaceScreen({
   const screenHeaderLeft = useMemo(() => <SidebarMenuToggle />, []);
 
   return (
-    <View style={styles.container}>
+    <FileDropZone style={styles.container}>
       <ScreenHeader left={screenHeaderLeft} borderless />
       <View style={contentStyle}>
         <TitlebarDragRegion />
@@ -2003,7 +2004,7 @@ export function NewWorkspaceScreen({
           {errorMessage ? <Text style={styles.errorText}>{errorMessage}</Text> : null}
         </ReanimatedAnimated.View>
       </View>
-    </View>
+    </FileDropZone>
   );
 }