Add image upload preview functionality; update titles and templates for consistency

Co-authored-by: Copilot <copilot@github.com>

frontend/app/static/style.css

@@ -359,6 +359,10 @@ pre {
   margin-top: 0.5rem;
 }
 
+.image-upload-preview {
+  margin-top: 0.75rem;
+}
+
 /* ─── Admin table ──────────────────────────────────────── */
 .stats-grid {
   display: grid;