fix: Fix runtime errors in mail app

- Fix envelopes list access: use index with bounds checking instead of .get()
- Add missing 'Any' type import to ContentContainer
- App now starts successfully without NameError or AttributeError

src/mail/app.py

@@ -270,7 +270,10 @@ class EmailViewerApp(App):
         metadata = self.message_store.get_metadata(message_id)
         envelope = None
         if metadata:
-            envelope = self.message_store.envelopes.get(metadata["index"])
+            index = metadata.get("index", 0)
+            # Check bounds before accessing envelopes list
+            if 0 <= index < len(self.message_store.envelopes):
+                envelope = self.message_store.envelopes[index]
 
         content_container.display_content(
             message_id, folder=folder, account=account, envelope=envelope