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/codex_config.rs

@@ -4,6 +4,7 @@ use std::path::PathBuf;
 use crate::config::{
     atomic_write, delete_file, sanitize_provider_name, write_json_file, write_text_file,
 };
+use crate::error::AppError;
 use serde_json::Value;
 use std::fs;
 use std::path::Path;
@@ -43,7 +44,7 @@ pub fn get_codex_provider_paths(
 }
 
 /// 删除 Codex 供应商配置文件
-pub fn delete_codex_provider_config(provider_id: &str, provider_name: &str) -> Result<(), String> {
+pub fn delete_codex_provider_config(provider_id: &str, provider_name: &str) -> Result<(), AppError> {
     let (auth_path, config_path) = get_codex_provider_paths(provider_id, Some(provider_name));
 
     delete_file(&auth_path).ok();
@@ -55,29 +56,23 @@ pub fn delete_codex_provider_config(provider_id: &str, provider_name: &str) -> R
 //（移除未使用的备份/保存/恢复/导入函数，避免 dead_code 告警）
 
 /// 原子写 Codex 的 `auth.json` 与 `config.toml`，在第二步失败时回滚第一步
-pub fn write_codex_live_atomic(auth: &Value, config_text_opt: Option<&str>) -> Result<(), String> {
+pub fn write_codex_live_atomic(auth: &Value, config_text_opt: Option<&str>) -> Result<(), AppError> {
     let auth_path = get_codex_auth_path();
     let config_path = get_codex_config_path();
 
     if let Some(parent) = auth_path.parent() {
-        std::fs::create_dir_all(parent)
-            .map_err(|e| format!("创建 Codex 目录失败: {}: {}", parent.display(), e))?;
+        std::fs::create_dir_all(parent).map_err(|e| AppError::io(parent, e))?;
     }
 
     // 读取旧内容用于回滚
     let old_auth = if auth_path.exists() {
-        Some(
-            fs::read(&auth_path)
-                .map_err(|e| format!("读取旧 auth.json 失败: {}: {}", auth_path.display(), e))?,
-        )
+        Some(fs::read(&auth_path).map_err(|e| AppError::io(&auth_path, e))?)
     } else {
         None
     };
     let _old_config =
         if config_path.exists() {
-            Some(fs::read(&config_path).map_err(|e| {
-                format!("读取旧 config.toml 失败: {}: {}", config_path.display(), e)
-            })?)
+            Some(fs::read(&config_path).map_err(|e| AppError::io(&config_path, e))?)
         } else {
             None
         };
@@ -88,13 +83,8 @@ pub fn write_codex_live_atomic(auth: &Value, config_text_opt: Option<&str>) -> R
         None => String::new(),
     };
     if !cfg_text.trim().is_empty() {
-        toml::from_str::<toml::Table>(&cfg_text).map_err(|e| {
-            format!(
-                "config.toml 语法错误: {} (路径: {})",
-                e,
-                config_path.display()
-            )
-        })?;
+        toml::from_str::<toml::Table>(&cfg_text)
+            .map_err(|e| AppError::toml(&config_path, e))?;
     }
 
     // 第一步：写 auth.json
@@ -115,43 +105,43 @@ pub fn write_codex_live_atomic(auth: &Value, config_text_opt: Option<&str>) -> R
 }
 
 /// 读取 `~/.codex/config.toml`，若不存在返回空字符串
-pub fn read_codex_config_text() -> Result<String, String> {
+pub fn read_codex_config_text() -> Result<String, AppError> {
     let path = get_codex_config_path();
     if path.exists() {
-        std::fs::read_to_string(&path).map_err(|e| format!("读取 config.toml 失败: {}", e))
+        std::fs::read_to_string(&path).map_err(|e| AppError::io(&path, e))
     } else {
         Ok(String::new())
     }
 }
 
 /// 从给定路径读取 config.toml 文本（路径存在时）；路径不存在则返回空字符串
-pub fn read_config_text_from_path(path: &Path) -> Result<String, String> {
+pub fn read_config_text_from_path(path: &Path) -> Result<String, AppError> {
     if path.exists() {
-        std::fs::read_to_string(path).map_err(|e| format!("读取 {} 失败: {}", path.display(), e))
+        std::fs::read_to_string(path).map_err(|e| AppError::io(path, e))
     } else {
         Ok(String::new())
     }
 }
 
 /// 对非空的 TOML 文本进行语法校验
-pub fn validate_config_toml(text: &str) -> Result<(), String> {
+pub fn validate_config_toml(text: &str) -> Result<(), AppError> {
     if text.trim().is_empty() {
         return Ok(());
     }
     toml::from_str::<toml::Table>(text)
         .map(|_| ())
-        .map_err(|e| format!("config.toml 语法错误: {}", e))
+        .map_err(|e| AppError::toml(Path::new("config.toml"), e))
 }
 
 /// 读取并校验 `~/.codex/config.toml`，返回文本（可能为空）
-pub fn read_and_validate_codex_config_text() -> Result<String, String> {
+pub fn read_and_validate_codex_config_text() -> Result<String, AppError> {
     let s = read_codex_config_text()?;
     validate_config_toml(&s)?;
     Ok(s)
 }
 
 /// 从指定路径读取并校验 config.toml，返回文本（可能为空）
-pub fn read_and_validate_config_from_path(path: &Path) -> Result<String, String> {
+pub fn read_and_validate_config_from_path(path: &Path) -> Result<String, AppError> {
     let s = read_config_text_from_path(path)?;
     validate_config_toml(&s)?;
     Ok(s)