Debug: add image preview (first page) with PyMuPDF/pdfium; return PDF path from debug flows; chain .then to render image

app.py

@@ -1512,11 +1512,11 @@ def debug_compile_uploaded_zip(zip_file):
             + _pdf_to_iframe_html(pdf_path, height="700px")
         )
         _write_logs(LOG_PATH, logs)
-        return html
+        return html, str(pdf_path)
     except Exception as e:
         logs.append(f"⚠️ preview failed: {e}")
         _write_logs(LOG_PATH, logs)
-        return f"<div>Compiled but preview failed: {e}</div>"
+        return f"<div>Compiled but preview failed: {e}</div>", None
 
 def debug_compile_output_zip():
     """Compile the repo-root output.zip (a real LaTeX project) and preview the resulting PDF."""