fix: use globalThis.fetch for image upload on Android

expoFetch does not support the React Native FormData { uri, name, type }
pattern. Switch upload request to globalThis.fetch which handles it
correctly. Also propagate upload errors instead of swallowing them.

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

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

@@ -77,33 +77,32 @@ export default function ComposeScreen() {
 
   const removeImage = () => setImageUri(null);
 
-  const uploadImage = async (): Promise<PostizUploadResult | null> => {
-    if (!imageUri) return null;
+  const uploadImage = async (): Promise<PostizUploadResult> => {
     setUploading(true);
     try {
       const formData = new FormData();
       if (Platform.OS === "web") {
-        const response = await expoFetch(imageUri);
+        const response = await expoFetch(imageUri!);
         const blob = await response.blob();
         formData.append("file", blob, "upload.jpg");
       } else {
         formData.append("file", {
-          uri: imageUri,
+          uri: imageUri!,
           name: "upload.jpg",
           type: "image/jpeg",
         } as unknown as Blob);
       }
-      const uploadRes = await expoFetch(`${baseUrl}/upload`, {
+      // eslint-disable-next-line no-undef
+      const uploadRes = await globalThis.fetch(`${baseUrl}/upload`, {
         method: "POST",
         headers: { Authorization: apiKey },
         body: formData,
       });
-      const data = await uploadRes.json() as PostizUploadResult;
-      return data;
-    } catch (e: unknown) {
-      const msg = e instanceof Error ? e.message : String(e);
-      Alert.alert("Upload Failed", `Could not upload image.\n${msg}`);
-      return null;
+      if (!uploadRes.ok) {
+        const raw = await uploadRes.text().catch(() => uploadRes.statusText);
+        throw new Error(`Upload HTTP ${uploadRes.status}: ${raw.slice(0, 200)}`);
+      }
+      return await uploadRes.json() as PostizUploadResult;
     } finally {
       setUploading(false);
     }
@@ -125,9 +124,7 @@ export default function ComposeScreen() {
       let media: Array<{ id: string; path: string }> = [];
       if (imageUri) {
         const uploaded = await uploadImage();
-        if (uploaded) {
-          media = [{ id: uploaded.id, path: uploaded.path }];
-        }
+        media = [{ id: uploaded.id, path: uploaded.path }];
       }
       const payload = {
         type: postNow ? "now" : "schedule",