refactor(backend): implement transaction mechanism and i18n errors for provider service

This commit completes phase 4 service layer extraction by introducing:

1. **Transaction mechanism with 2PC (Two-Phase Commit)**:
   - Introduced `run_transaction()` wrapper with snapshot-based rollback
   - Implemented `LiveSnapshot` enum to capture and restore live config files
   - Added `PostCommitAction` to separate config.json persistence from live file writes
   - Applied to critical operations: add, update, switch providers
   - Ensures atomicity: memory + config.json + live files stay consistent

2. **Internationalized error handling**:
   - Added `AppError::Localized` variant with key + zh + en messages
   - Implemented `AppError::localized()` helper function
   - Migrated 24 error sites to use i18n-ready errors
   - Enables frontend to display errors in user's preferred language

3. **Concurrency optimization**:
   - Fixed `get_custom_endpoints()` to use read lock instead of write lock
   - Ensured async IO operations (usage query) execute outside lock scope
   - Added defensive RAII lock management with explicit scope blocks

4. **Code organization improvements**:
   - Reduced commands/provider.rs from ~800 to ~320 lines (-60%)
   - Expanded services/provider.rs with transaction infrastructure
   - Added unit tests for validation and credential extraction
   - Documented legacy file cleanup logic with inline comments

5. **Backfill mechanism refinement**:
   - Ensured live config is synced back to memory before switching
   - Maintains SSOT (Single Source of Truth) architecture principle
   - Handles Codex dual-file (auth.json + config.toml) atomically

Breaking changes: None (internal refactoring only)
Performance: Improved read concurrency, no measurable overhead from snapshots
Test coverage: Added validation tests, updated service layer tests

src-tauri/tests/mcp_commands.rs

@@ -76,6 +76,10 @@ fn import_default_config_without_live_file_returns_error() {
     let err = import_default_config_test_hook(&state, AppType::Claude)
         .expect_err("missing live file should error");
     match err {
+        AppError::Localized { zh, .. } => assert!(
+            zh.contains("Claude Code 配置文件不存在"),
+            "unexpected error message: {zh}"
+        ),
         AppError::Message(msg) => assert!(
             msg.contains("Claude Code 配置文件不存在"),
             "unexpected error message: {msg}"