feat(editor): implement deferred image uploading and pending state management

Refactor the image handling workflow to decouple image selection from the upload process. This improves user experience by allowing users to continue composing posts or trades while images are queued for upload, and increases reliability by using a centralized pending image utility.

- **Deferred Uploads**: Replaced immediate image uploads in `CreatePostScreen`, `PostDetailScreen`, and `CreateTradeScreen` with a "pending" state. Images are now uploaded in bulk during the final submission phase.
- **BlockEditor Enhancements**:
    - Updated `useBlockEditor` to support asynchronous batch uploading of all pending images via a new `uploadPendingImages` method.
    - Removed inline uploading overlays in favor of a more robust state-driven approach.
    - Improved `BlockEditorHandle` to expose block retrieval and batch upload capabilities.
- **API Layer Improvements**:
    - Migrated native image uploads from standard `FormData` to `expo-file-system`'s multipart upload to resolve compatibility issues with recent React Native versions.
    - Maintained `fetch` + `Blob` logic for web platform compatibility.
- **New Utilities**: Introduced `src/utils/pendingImages.ts` to manage the lifecycle of local (pending) vs. remote (uploaded) images across the application.

src/components/business/BlockEditor/BlockEditor.tsx

@@ -14,7 +14,8 @@ export interface BlockEditorHandle {
   insertTextAtCursor: (text: string) => void;
   getSegments: () => MessageSegment[];
   getContent: () => string;
-  hasUploadingImages: () => boolean;
+  getBlocks: () => EditorBlock[];
+  uploadPendingImages: () => Promise<boolean>;
   focus: () => void;
 }
 
@@ -38,7 +39,8 @@ const BlockEditor = React.forwardRef<BlockEditorHandle, BlockEditorProps>(
       insertTextAtCursor: editor.insertTextAtCursor,
       getSegments: editor.getSegments,
       getContent: editor.getContent,
-      hasUploadingImages: editor.hasUploadingImages,
+      getBlocks: editor.getBlocks,
+      uploadPendingImages: editor.uploadPendingImages,
       focus: () => {
         const firstTextBlock = editor.blocks.find(b => b.type === 'text');
         if (firstTextBlock) {