fix(mobile): restore images on repost, improve media library 404 error, null-safe stripHtml

- Pass post.image[] as JSON prefillImages param when prefilling compose from an existing post (repost/edit/retry), replacing the broken single-image prefillImagePath/prefillImageId approach
- MediaLibraryModal now shows the attempted URL and a clear explanation on 404, making it easier to diagnose if the Postiz version does not expose GET /media
- stripHtml accepts null/undefined input and returns "" instead of throwing

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

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

@@ -152,13 +152,14 @@ export default function PostsScreen() {
 
   const handlePrefillCompose = (post: PostizPost) => {
     const integrations = post.integrations ?? (post.integration ? [post.integration] : []);
-    router.push({
-      pathname: "/(tabs)/compose",
-      params: {
-        prefillContent: stripHtml(post.content),
-        prefillIntegrationIds: integrations.map((i) => i.id).join(","),
-      },
-    });
+    const params: Record<string, string> = {
+      prefillContent: stripHtml(post.content),
+      prefillIntegrationIds: integrations.map((i) => i.id).join(","),
+    };
+    if (post.image?.length) {
+      params.prefillImages = JSON.stringify(post.image);
+    }
+    router.push({ pathname: "/(tabs)/compose", params });
   };
 
   const startReschedule = (post: PostizPost) => {