fix: executor subtitle_writer leak + adapter Path/None-default fixes

- executor: manage subtitle_writer via ExitStack (stack.callback)
- executor: remove manual subtitle_writer.close()
- WebUI adapter: wrap source_path, output_folder, cover_image_path in Path()
- WebUI adapter: remove unused threading/time imports
- PyQt adapter: fix getattr(attr, None) or default for 4 fields

abogen/application/conversion_executor.py

@@ -134,6 +134,7 @@ def execute_conversion(
             )
             if subtitle_writer:
                 subtitle_writer.open()
+                stack.callback(subtitle_writer.close)
                 result.subtitle_paths.append(subtitle_writer.path)
 
         # Chapter directory
@@ -381,10 +382,6 @@ def execute_conversion(
             )
             events.log("Outro synthesized.")
 
-        # Close subtitle writer
-        if subtitle_writer:
-            subtitle_writer.close()
-
     # Set result metadata
     result.total_chapters = len(plan.chapters)
     result.total_segments = sum(len(ch.segments) for ch in plan.chapters)