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/composer/draft/workspace-tab.tsx

@@ -8,6 +8,7 @@ import { useKeyboardShiftStyle } from "@/hooks/use-keyboard-shift-style";
 import { useContainerWidthBelow } from "@/hooks/use-container-width";
 import invariant from "tiny-invariant";
 import { Composer } from "@/composer";
+import { FileDropZone } from "@/components/file-drop/file-drop-zone";
 import { DraftAgentModeControl } from "@/composer/agent-controls/mode-control";
 import { ComposerImportPill } from "@/composer/draft/import-pill";
 import { AgentStreamView } from "@/agent-stream/view";
@@ -644,7 +645,7 @@ export function WorkspaceDraftAgentTab({
   );
 
   return (
-    <View style={styles.container}>
+    <FileDropZone style={styles.container}>
       <View style={styles.contentContainer}>
         {isSubmitting && draftAgent ? (
           <View style={styles.streamContainer}>
@@ -702,7 +703,7 @@ export function WorkspaceDraftAgentTab({
           isCompactLayout={isCompactComposerLayout}
         />
       </ReanimatedAnimated.View>
-    </View>
+    </FileDropZone>
   );
 }