refactor(backend): phase 4 - extract provider service layer

Architecture improvements:
- Extract ProviderService with switch/backfill/write methods
- Reduce command layer from 160 to 13 lines via delegation
- Separate business logic (services) from state management (commands)
- Introduce precise error handling with structured validation

Refactoring details:
- Split Codex/Claude switching into symmetric private methods
- Add multi-layer validation for Codex auth field (existence + type)
- Extract import_config_from_path for command and test reuse
- Expose export_config_to_file and ProviderService in public API

Test coverage:
- Add 10+ integration tests for Claude/Codex switching flows
- Cover import/export success and failure scenarios (JSON parse, missing file)
- Verify state consistency on error paths (current remains unchanged)
- Test snapshot backfill for both old and new providers after switching

src-tauri/src/lib.rs

@@ -13,17 +13,21 @@ mod provider;
 mod settings;
 mod speedtest;
 mod store;
+mod services;
 mod usage_script;
 
 pub use app_config::{AppType, MultiAppConfig};
 pub use codex_config::{get_codex_auth_path, get_codex_config_path, write_codex_live_atomic};
 pub use config::{get_claude_mcp_path, get_claude_settings_path, read_json_file};
-pub use import_export::{create_backup, sync_current_providers_to_live};
+pub use import_export::{
+    create_backup, export_config_to_file, import_config_from_path, sync_current_providers_to_live,
+};
 pub use provider::Provider;
 pub use settings::{update_settings, AppSettings};
 pub use mcp::{import_from_claude, import_from_codex, sync_enabled_to_claude, sync_enabled_to_codex};
 pub use error::AppError;
 pub use store::AppState;
+pub use services::ProviderService;
 pub use commands::*;
 
 use tauri::{