Backend: Add Anthropic/Claude provider integration

- Add anthropic.rs module with streaming support
- Convert between internal and Anthropic tool/message formats
- Add keyring dependency for secure API key storage
- Add API key management commands (get_exists, set)
- Auto-detect provider from model name (claude-* prefix)
- Support SSE streaming from Anthropic API
- Handle tool calling with Anthropic's format
- Add cancellation support for Anthropic streams

src-tauri/src/llm/mod.rs

@@ -1,3 +1,4 @@
+pub mod anthropic;
 pub mod ollama;
 pub mod prompts;
 pub mod types;