fix: Restore embedding-mode parameter to all examples

- All examples now have --embedding-mode parameter (unified interface benefit)
- Default is 'sentence-transformers' (consistent with original behavior)
- Users can now use OpenAI or MLX embeddings with any data source
- Maintains functional equivalence with original scripts

examples/email_rag.py

@@ -20,8 +20,7 @@ class EmailRAG(BaseRAGExample):
         super().__init__(
             name="Email",
             description="Process and query Apple Mail emails with LEANN",
-            default_index_name="mail_index",  # Match original: "./mail_index"
-            include_embedding_mode=False,  # Original mail_reader_leann.py doesn't have embedding_mode
+            default_index_name="mail_index",
         )
 
     def _add_specific_arguments(self, parser):