refactor(backend): phase 1 - unified error handling with thiserror

Introduce AppError enum to replace Result<T, String> pattern across
the codebase, improving error context preservation and type safety.

## Changes

### Core Infrastructure
- Add src/error.rs with AppError enum using thiserror
- Add thiserror dependency to Cargo.toml
- Implement helper functions: io(), json(), toml() for ergonomic error creation
- Implement From<PoisonError> for automatic lock error conversion
- Implement From<AppError> for String to maintain Tauri command compatibility

### Module Migrations (60% complete)
- config.rs: Full migration to AppError
  - read_json_file, write_json_file, atomic_write
  - archive_file, copy_file, delete_file
- claude_mcp.rs: Full migration to AppError
  - get_mcp_status, read_mcp_json, upsert_mcp_server
  - delete_mcp_server, validate_command_in_path
  - set_mcp_servers_map
- codex_config.rs: Full migration to AppError
  - write_codex_live_atomic with rollback support
  - read_and_validate_codex_config_text
  - validate_config_toml
- app_config.rs: Partial migration
  - MultiAppConfig::load, MultiAppConfig::save
- store.rs: Partial migration
  - AppState::save now returns Result<(), AppError>
- commands.rs: Minimal changes
  - Use .map_err(Into::into) for compatibility
- mcp.rs: Minimal changes
  - sync_enabled_to_claude uses Into::into conversion

### Documentation
- Add docs/BACKEND_REFACTOR_PLAN.md with detailed refactoring roadmap

## Benefits
- Type-safe error handling with preserved error chains
- Better error messages with file paths and context
- Reduced boilerplate code (118 Result<T, String> instances to migrate)
- Automatic error conversion for seamless integration

## Testing
- All existing tests pass (4/4)
- Compilation successful with no warnings
- Build time: 0.61s (no performance regression)

## Remaining Work
- claude_plugin.rs (7 functions)
- migration.rs, import_export.rs
- Add unit tests for error.rs
- Complete commands.rs migration after dependent modules

Co-authored-by: Claude <claude@anthropic.com>

src-tauri/src/app_config.rs

@@ -19,6 +19,7 @@ pub struct McpRoot {
 }
 
 use crate::config::{copy_file, get_app_config_dir, get_app_config_path, write_json_file};
+use crate::error::AppError;
 use crate::provider::ProviderManager;
 
 /// 应用类型
@@ -80,7 +81,7 @@ impl Default for MultiAppConfig {
 
 impl MultiAppConfig {
     /// 从文件加载配置（处理v1到v2的迁移）
-    pub fn load() -> Result<Self, String> {
+    pub fn load() -> Result<Self, AppError> {
         let config_path = get_app_config_path();
 
         if !config_path.exists() {
@@ -90,7 +91,7 @@ impl MultiAppConfig {
 
         // 尝试读取文件
         let content = std::fs::read_to_string(&config_path)
-            .map_err(|e| format!("读取配置文件失败: {}", e))?;
+            .map_err(|e| AppError::io(&config_path, e))?;
 
         // 检查是否是旧版本格式（v1）
         if let Ok(v1_config) = serde_json::from_str::<ProviderManager>(&content) {
@@ -130,11 +131,11 @@ impl MultiAppConfig {
         }
 
         // 尝试读取v2格式
-        serde_json::from_str::<Self>(&content).map_err(|e| format!("解析配置文件失败: {}", e))
+        serde_json::from_str::<Self>(&content).map_err(|e| AppError::json(&config_path, e))
     }
 
     /// 保存配置到文件
-    pub fn save(&self) -> Result<(), String> {
+    pub fn save(&self) -> Result<(), AppError> {
         let config_path = get_app_config_path();
         // 先备份旧版（若存在）到 ~/.cc-switch/config.json.bak，再写入新内容
         if config_path.exists() {