feat: official Postiz icon + strip HTML from post content display

- Replace icon.png with official Postiz logo (1024x1024, generated from
  upstream postiz.svg at gitroomhq/postiz-app)
- Add lib/stripHtml.ts: converts <br>/<p> to newlines, strips all tags,
  decodes HTML entities
- PostCard: use stripHtml on content before truncation and display
- posts.tsx: use stripHtml for context menu preview and clipboard copy
  (API payloads keep original HTML for retry/reschedule)

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

artifacts/postiz-mobile/lib/stripHtml.ts

@@ -0,0 +1,14 @@
+export function stripHtml(html: string): string {
+  return html
+    .replace(/<br\s*\/?>/gi, "\n")
+    .replace(/<\/p>/gi, "\n")
+    .replace(/<[^>]+>/g, "")
+    .replace(/&amp;/g, "&")
+    .replace(/&lt;/g, "<")
+    .replace(/&gt;/g, ">")
+    .replace(/&quot;/g, '"')
+    .replace(/&#39;/g, "'")
+    .replace(/&nbsp;/g, " ")
+    .replace(/\n{3,}/g, "\n\n")
+    .trim();
+}