fix: always include image array in post value to satisfy Postiz API validation

Postiz requires posts[].value[].image to always be an array. Omitting it
when no image is selected was causing a 400 Bad Request.

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

artifacts/postiz-mobile/app/(tabs)/compose.tsx

@@ -144,13 +144,9 @@ export default function ComposeScreen() {
         shortLink: false,
         tags: [] as string[],
         posts: selectedChannels.map((integrationId) => {
-          const valueItem: { content: string; image?: Array<{ id: string; path: string }> } = {
-            content: content.trim(),
-          };
-          if (media.length > 0) valueItem.image = media;
           return {
             integration: { id: integrationId },
-            value: [valueItem],
+            value: [{ content: content.trim(), image: media }],
           };
         }),
       };