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/components/MediaLibraryModal.tsx

@@ -66,12 +66,20 @@ export function MediaLibraryModal({ visible, workspaces, defaultWorkspaceId, max
     if (!activeWorkspace) return;
     setLoading(true);
     setError(null);
+    const url = `${activeWorkspace.baseUrl}/media`;
     try {
       // eslint-disable-next-line no-undef
-      const res = await globalThis.fetch(`${activeWorkspace.baseUrl}/media`, {
+      const res = await globalThis.fetch(url, {
         headers: { Authorization: activeWorkspace.apiKey },
       });
-      if (!res.ok) throw new Error(`HTTP ${res.status}`);
+      if (!res.ok) {
+        if (res.status === 404) {
+          throw new Error(
+            `Media listing endpoint not found (404).\nURL tried: ${url}\n\nThis feature requires Postiz to expose GET /media in its public API. Your version may not support it yet.`
+          );
+        }
+        throw new Error(`HTTP ${res.status} — ${url}`);
+      }
       const data = await res.json();
       const list: RawMediaItem[] = Array.isArray(data)
         ? data