refactor(Post, PostMapper, PostRepository, CreatePostScreen, HomeScreen): update communityId to channelId for improved clarity

- Renamed communityId to channelId across Post entity, PostMapper, and PostRepository for consistency and clarity.
- Updated CreatePostScreen to include channel selection functionality, enhancing user experience when creating posts.
- Adjusted HomeScreen to support filtering posts by channel, improving content organization.
- Refactored related interfaces and services to align with the new channelId terminology, ensuring a cohesive codebase.

src/services/api.ts

@@ -8,6 +8,7 @@
 import AsyncStorage from '@react-native-async-storage/async-storage';
 import { CommonActions } from '@react-navigation/native';
 import Constants from 'expo-constants';
+import { Platform } from 'react-native';
 
 // 生产地址 https://bbs.littlelan.cn
 const getBaseUrl = () => {
@@ -381,12 +382,20 @@ class ApiClient {
   ): Promise<ApiResponse<T>> {
     const formData = new FormData();
     
-    // 添加文件（使用image字段名）
-    formData.append('image', {
-      uri: file.uri,
-      name: file.name,
-      type: file.type,
-    } as any);
+    // 添加文件（使用 image 字段名）
+    // Web 端需要 append Blob/File；原生端可以直接传 { uri, name, type }。
+    if (Platform.OS === 'web') {
+      const fileResponse = await fetch(file.uri);
+      const blob = await fileResponse.blob();
+      const uploadFile = new File([blob], file.name, { type: file.type || blob.type || 'application/octet-stream' });
+      formData.append('image', uploadFile);
+    } else {
+      formData.append('image', {
+        uri: file.uri,
+        name: file.name,
+        type: file.type,
+      } as any);
+    }
 
     // 添加额外数据
     if (additionalData) {