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/channelService.ts

@@ -0,0 +1,29 @@
+import { api } from './api';
+
+export interface ChannelItem {
+  id: string;
+  name: string;
+  slug: string;
+  description?: string;
+  sort_order: number;
+  is_active: boolean;
+}
+
+interface ChannelListResponse {
+  list: ChannelItem[];
+}
+
+class ChannelService {
+  async list(): Promise<ChannelItem[]> {
+    try {
+      const res = await api.get<ChannelListResponse>('/channels');
+      return res.data?.list || [];
+    } catch (error) {
+      console.error('[channelService] 获取频道失败:', error);
+      return [];
+    }
+  }
+}
+
+export const channelService = new ChannelService();
+