fix: revert incorrect Bearer prefix on Authorization header

Commit 39d5e5d added `Bearer ${apiKey}` to the axios client but this
Postiz instance expects the raw API key with no prefix. Reverting to
the original format that was confirmed working in the initial commit.
Same fix applied to the image upload header in compose.tsx.

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

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

@@ -95,7 +95,7 @@ export default function ComposeScreen() {
       }
       const uploadRes = await expoFetch(`${baseUrl}/upload`, {
         method: "POST",
-        headers: { Authorization: `Bearer ${apiKey}` },
+        headers: { Authorization: apiKey },
         body: formData,
       });
       const data = await uploadRes.json() as PostizUploadResult;