Add chat support for invisible interface in Bluesky buffers

ChatBuffer now has get_message(), get_formatted_message(), and view_item()
so repeat_item, copy_to_clipboard, and view work correctly with chat
messages. open_conversation (Ctrl+Win+Alt+C) detects chat buffers and
delegates to view_chat(). Opening a chat buffer now announces its name
and item count for screen reader feedback.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/controller/blueski/handler.py

@@ -554,6 +554,10 @@ class Handler:
 
     def open_conversation(self, controller, buffer):
         """Standard action for Control+Win+C"""
+        # If this is a chat conversation list, open the selected chat
+        if buffer.type == "chat" and hasattr(buffer, "view_chat"):
+            buffer.view_chat()
+            return
         item = buffer.get_item()
         if not item:
             return