fix: improve POST payload and error reporting in compose

- Omit image field from content when no image is selected (sending
  image:[] likely fails the API's schema validation with a 400)
- Extract full axios response body in the error alert so the actual
  API error message is visible instead of just the HTTP status line

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

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

@@ -1,5 +1,6 @@
 import { Feather } from "@expo/vector-icons";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
+import axios from "axios";
 import DateTimePicker from "@react-native-community/datetimepicker";
 import * as Haptics from "expo-haptics";
 import { Image } from "expo-image";
@@ -129,10 +130,15 @@ export default function ComposeScreen() {
           media = [{ id: uploaded.id, path: uploaded.path }];
         }
       }
+      const contentItem: { content: string; image?: Array<{ id: string; path: string }> } = {
+        content: content.trim(),
+      };
+      if (media.length > 0) contentItem.image = media;
+
       const payload = {
         type: postNow ? "now" : "schedule",
         date: postNow ? new Date().toISOString() : scheduleDate.toISOString(),
-        content: [{ content: content.trim(), image: media }],
+        content: [contentItem],
         integrations: selectedChannels,
       };
       await client.post("posts", payload);
@@ -147,7 +153,15 @@ export default function ComposeScreen() {
     } catch (e: unknown) {
       Haptics.notificationAsync(Haptics.NotificationFeedbackType.Error);
       let msg = "Could not submit post.";
-      if (e instanceof Error) msg += `\n${e.message}`;
+      if (axios.isAxiosError(e)) {
+        const data = e.response?.data;
+        const detail = data
+          ? (typeof data === "string" ? data : (data?.message ?? data?.error ?? JSON.stringify(data))).toString().slice(0, 300)
+          : e.message;
+        msg = `HTTP ${e.response?.status ?? "?"}: ${detail}`;
+      } else if (e instanceof Error) {
+        msg += `\n${e.message}`;
+      }
       Alert.alert("Failed", msg);
     } finally {
       setSubmitting(false);