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/lib/stripHtml.ts

@@ -1,4 +1,5 @@
-export function stripHtml(html: string): string {
+export function stripHtml(html: string | null | undefined): string {
+  if (!html) return "";
   // Decode entities first so encoded tags like <p> are also stripped
   let s = html
     .replace(/&/g, "&")