refactor(proxy): extract format transform into standalone module

- Create new `proxy/transform/` module with clean architecture:
  - `traits.rs`: FormatTransformer trait definition
  - `format.rs`: ApiFormat enum (Anthropic, OpenAI, Gemini)
  - `registry.rs`: TransformerRegistry with global instance
  - `config.rs`: TransformConfig from Provider settings
  - `anthropic_openai/`: Bidirectional Anthropic ↔ OpenAI converters

- Move streaming transform from providers/ to transform/
- Remove legacy transform code from providers/adapter.rs and claude.rs
- Update handlers.rs to use new transform module

This refactor improves maintainability and makes it easier to add
new format converters in the future.

src-tauri/src/proxy/mod.rs

@@ -22,6 +22,7 @@ pub mod response_processor;
 pub(crate) mod server;
 pub mod session;
 pub mod thinking_rectifier;
+pub mod transform;
 pub(crate) mod types;
 pub mod usage;