feat: add long-press contextual actions on post cards

Long press any post card to open a context menu with state-aware actions:
- Copy text (all states)
- ERROR: Retry now, Edit & retry, View error message
- QUEUE: Edit, Reschedule (native DateTimePicker → PUT /posts/:id)
- PUBLISHED: Repost
- DRAFT: Edit & schedule

Compose screen now accepts prefillContent/prefillIntegrationIds router
params to pre-fill content and channel selection when editing or reposting.
Adds expo-clipboard for clipboard support.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

artifacts/postiz-mobile/context/PostizContext.tsx

@@ -35,6 +35,7 @@ export interface PostizPost {
   integrations?: PostizIntegration[];
   image?: PostizMediaItem[];
   group?: string;
+  errorMessage?: string;
 }
 
 export interface PostizUploadResult {