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/panels/agent-panel.tsx

@@ -12,6 +12,7 @@ import { shallow, useShallow } from "zustand/shallow";
 import { useStoreWithEqualityFn } from "zustand/traditional";
 import { AgentStreamView, type AgentStreamViewHandle } from "@/agent-stream/view";
 import { ArchivedAgentCallout } from "@/components/archived-agent-callout";
+import { FileDropZone } from "@/components/file-drop/file-drop-zone";
 import { Composer } from "@/composer";
 import { AgentModeControl } from "@/composer/agent-controls/mode-control";
 import { RewindComposerRestoreProvider } from "@/components/rewind/composer-restore";
@@ -1177,7 +1178,7 @@ const ChatAgentReadyContent = memo(function ChatAgentReadyContent({
   return (
     <RewindComposerRestoreProvider text={agentInputDraft.text} setText={agentInputDraft.setText}>
       <View style={styles.root}>
-        <View style={styles.container}>
+        <FileDropZone style={styles.container} disabled={isArchivingCurrentAgent}>
           {contentContainer}
 
           {composerSection}
@@ -1189,7 +1190,7 @@ const ChatAgentReadyContent = memo(function ChatAgentReadyContent({
           ) : null}
 
           <ToastViewport toast={toast} onDismiss={dismiss} placement="panel" />
-        </View>
+        </FileDropZone>
 
         {isArchivingCurrentAgent ? (
           <View style={styles.archivingOverlay} testID="agent-archiving-overlay">