feat(provider): Add MiniMax AI provider integration

- Add minimax to PROVIDERS lists in gui_2.py and app_controller.py
- Add minimax credentials template in ai_client.py
- Implement _list_minimax_models, _classify_minimax_error, _ensure_minimax_client
- Implement _send_minimax with streaming and reasoning support
- Add minimax to send(), list_models(), reset_session(), get_history_bleed_stats()
- Add unit tests in tests/test_minimax_provider.py

config.toml

@@ -1,6 +1,6 @@
 [ai]
-provider = "deepseek"
-model = "deepseek-v3"
+provider = "gemini"
+model = "gemini-2.5-flash-lite"
 temperature = 0.0
 max_tokens = 8192
 history_trunc_limit = 8000