Improve code style in load_state

Replace duplicate strip() calls with filter(None, generator) for
cleaner and more efficient processing of processed Message-IDs.

Changes:
- Use set(filter(None, (line.strip() for line in f))) to avoid calling
  strip() twice and filter empty lines

main.py

@@ -155,7 +155,7 @@ class EmailForwarder:
         processed_file = self.config["processed_file"]
         if os.path.exists(processed_file):
             with open(processed_file, "r") as f:
-                self.processed_ids = set(line.strip() for line in f if line.strip())
+                self.processed_ids = set(filter(None, (line.strip() for line in f)))
 
     def save_state(self):
         """Save last run and processed IDs."""