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/commands.rs

@@ -546,10 +546,7 @@ pub async fn import_default_config(
             }
             let auth: serde_json::Value =
                 crate::config::read_json_file::<serde_json::Value>(&auth_path)?;
-            let config_str = match crate::codex_config::read_and_validate_codex_config_text() {
-                Ok(s) => s,
-                Err(e) => return Err(e),
-            };
+            let config_str = crate::codex_config::read_and_validate_codex_config_text()?;
             serde_json::json!({ "auth": auth, "config": config_str })
         }
         AppType::Claude => {
@@ -770,31 +767,31 @@ pub async fn open_app_config_folder(handle: tauri::AppHandle) -> Result<bool, St
 /// 获取 Claude MCP 状态（settings.local.json 与 mcp.json）
 #[tauri::command]
 pub async fn get_claude_mcp_status() -> Result<crate::claude_mcp::McpStatus, String> {
-    claude_mcp::get_mcp_status()
+    claude_mcp::get_mcp_status().map_err(Into::into)
 }
 
 /// 读取 mcp.json 文本内容（不存在则返回 Ok(None)）
 #[tauri::command]
 pub async fn read_claude_mcp_config() -> Result<Option<String>, String> {
-    claude_mcp::read_mcp_json()
+    claude_mcp::read_mcp_json().map_err(Into::into)
 }
 
 /// 新增或更新一个 MCP 服务器条目
 #[tauri::command]
 pub async fn upsert_claude_mcp_server(id: String, spec: serde_json::Value) -> Result<bool, String> {
-    claude_mcp::upsert_mcp_server(&id, spec)
+    claude_mcp::upsert_mcp_server(&id, spec).map_err(Into::into)
 }
 
 /// 删除一个 MCP 服务器条目
 #[tauri::command]
 pub async fn delete_claude_mcp_server(id: String) -> Result<bool, String> {
-    claude_mcp::delete_mcp_server(&id)
+    claude_mcp::delete_mcp_server(&id).map_err(Into::into)
 }
 
 /// 校验命令是否在 PATH 中可用（不执行）
 #[tauri::command]
 pub async fn validate_mcp_command(cmd: String) -> Result<bool, String> {
-    claude_mcp::validate_command_in_path(&cmd)
+    claude_mcp::validate_command_in_path(&cmd).map_err(Into::into)
 }
 
 // =====================