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

app.py

@@ -1859,7 +1859,7 @@ def debug_compile_output_zip():
     except Exception as e:
         logs.append(f"❌ unzip failed: {e}")
         _write_logs(LOG_PATH, logs)
-        return "<div style='color:#b00'>Unzip failed.</div>"
+        return "<div style='color:#b00'>Unzip failed.</div>", None
 
     # Locate poster_output.tex (fallback to poster.tex)
     tex_path = None