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

app.py

@@ -2875,7 +2875,8 @@ def debug_compile_output_zip():
     if not zip_path.exists():
         return (
             "<div style='color:#b00'><b>output.zip not found at repo root.</b></div>"
-            + f"<div>Expected at: {zip_path}</div>"
+            + f"<div>Expected at: {zip_path}</div>",
+            None,
         )
 
     # Prepare workspace