feat: add unified endpoint speed test for API providers

Add a comprehensive endpoint latency testing system that allows users to:
- Test multiple API endpoints concurrently
- Auto-select the fastest endpoint based on latency
- Add/remove custom endpoints dynamically
- View latency results with color-coded indicators

Backend (Rust):
- Implement parallel HTTP HEAD requests with configurable timeout
- Handle various error scenarios (timeout, connection failure, invalid URL)
- Return structured latency data with status codes

Frontend (React):
- Create interactive speed test UI component with auto-sort by latency
- Support endpoint management (add/remove custom endpoints)
- Extract and update Codex base_url from TOML configuration
- Integrate with provider presets for default endpoint candidates

This feature improves user experience when selecting optimal API endpoints,
especially useful for users with multiple provider options or proxy setups.

src-tauri/Cargo.toml

@@ -29,6 +29,9 @@ tauri-plugin-updater = "2"
 tauri-plugin-dialog = "2"
 dirs = "5.0"
 toml = "0.8"
+reqwest = { version = "0.12", default-features = false, features = ["rustls-tls"] }
+tokio = { version = "1", features = ["macros", "rt-multi-thread", "time"] }
+futures = "0.3"
 
 [target.'cfg(any(target_os = "macos", target_os = "windows", target_os = "linux"))'.dependencies]
 tauri-plugin-single-instance = "2"