fix: strip HTML-encoded tags (decode entities before stripping)

The previous stripHtml decoded </> after the regex pass, so content
stored as <p>text</p> was never stripped. Now entities are
decoded first, then all tags are removed.

Also strip HTML when prefilling compose from an existing post (Edit/Repost)
so the text field shows clean content, not raw markup.

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

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

@@ -155,7 +155,7 @@ export default function PostsScreen() {
     router.push({
       pathname: "/(tabs)/compose",
       params: {
-        prefillContent: post.content,
+        prefillContent: stripHtml(post.content),
         prefillIntegrationIds: integrations.map((i) => i.id).join(","),
       },
     });