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/attachments/drop.ts

@@ -4,7 +4,7 @@ import {
   isRasterImagePath,
 } from "@/attachments/file-types";
 import { readDesktopFileBytes, type PickedFile } from "@/attachments/picked-file";
-import type { DroppedItem } from "@/hooks/use-file-drop-zone";
+import type { DroppedItem } from "@/components/file-drop/types";
 
 interface DroppedAttachmentsRuntime {
   readDesktopFileBytes(path: string): Promise<Uint8Array>;