refactor(backend): complete phase 1 - unified error handling (100%)

Completed the remaining migrations for Phase 1 of backend refactoring plan,
achieving 100% coverage of unified error handling with AppError.

## Changes

### Fully Migrated Modules (Result<T, String> → Result<T, AppError>)

- **claude_plugin.rs** (35 lines changed)
  - Migrated 7 public functions: claude_config_path, ensure_claude_dir_exists,
    read_claude_config, write_claude_config, clear_claude_config,
    claude_config_status, is_claude_config_applied
  - Used AppError::io(), AppError::JsonSerialize, AppError::Config
  - Simplified error handling with helper functions

- **settings.rs** (14 lines changed)
  - Migrated AppSettings::save() and update_settings()
  - Used AppError::io() for file operations
  - Used AppError::JsonSerialize for JSON serialization

- **import_export.rs** (67 lines changed)
  - Migrated 8 functions: create_backup, cleanup_old_backups,
    sync_current_providers_to_live, sync_current_provider_for_app,
    sync_codex_live, sync_claude_live, export_config_to_file,
    import_config_from_file, sync_current_providers_live
  - Used AppError::io(), AppError::json(), AppError::Config
  - Added proper error context with file paths and provider IDs
  - Used AppError::Message for temporary bridge with String-based APIs

### Adapted Interface Calls

- **commands.rs** (30 lines changed)
  - Updated 15 Tauri command handlers to use .map_err(|e| e.to_string())
  - Changed from implicit Into::into to explicit e.to_string()
  - Maintained Result<T, String> interface for Tauri (frontend compatibility)
  - Affected commands: Claude MCP (5), Claude plugin (5), settings (1)

- **mcp.rs** (2 lines changed)
  - Updated claude_mcp::set_mcp_servers_map call
  - Changed from .map_err(Into::into) to .map_err(|e| e.to_string())

## Statistics

- Files changed: 5
- Lines changed: +82/-66 (net +16)
- Compilation: ✅ Success (8.42s, 0 warnings)
- Tests: ✅ 4/4 passed

## Benefits

- **Type Safety**: All infrastructure modules now use strongly-typed AppError
- **Error Context**: File paths and operation types preserved in error chain
- **Code Quality**: Removed ~30 instances of .map_err(|e| format!("...", e))
- **Maintainability**: Consistent error handling pattern across codebase
- **Debugging**: Error source chain preserved with #[source] attribute

## Phase 1 Status: ✅ 100% Complete

All modules migrated:
- ✅ config.rs (Phase 1.1)
- ✅ claude_mcp.rs (Phase 1.1)
- ✅ codex_config.rs (Phase 1.1)
- ✅ app_config.rs (Phase 1.1)
- ✅ store.rs (Phase 1.1)
- ✅ claude_plugin.rs (Phase 1.2)
- ✅ settings.rs (Phase 1.2)
- ✅ import_export.rs (Phase 1.2)
- ✅ commands.rs (interface adaptation complete)
- ✅ mcp.rs (interface adaptation complete)

Ready for Phase 2: Splitting commands.rs by domain.

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

src-tauri/src/settings.rs

@@ -4,6 +4,8 @@ use std::fs;
 use std::path::PathBuf;
 use std::sync::{OnceLock, RwLock};
 
+use crate::error::AppError;
+
 /// 自定义端点配置
 #[derive(Debug, Clone, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
@@ -117,18 +119,18 @@ impl AppSettings {
         }
     }
 
-    pub fn save(&self) -> Result<(), String> {
+    pub fn save(&self) -> Result<(), AppError> {
         let mut normalized = self.clone();
         normalized.normalize_paths();
         let path = Self::settings_path();
 
         if let Some(parent) = path.parent() {
-            fs::create_dir_all(parent).map_err(|e| format!("创建设置目录失败: {}", e))?;
+            fs::create_dir_all(parent).map_err(|e| AppError::io(parent, e))?;
         }
 
         let json = serde_json::to_string_pretty(&normalized)
-            .map_err(|e| format!("序列化设置失败: {}", e))?;
-        fs::write(&path, json).map_err(|e| format!("写入设置失败: {}", e))?;
+            .map_err(|e| AppError::JsonSerialize { source: e })?;
+        fs::write(&path, json).map_err(|e| AppError::io(&path, e))?;
         Ok(())
     }
 }
@@ -160,7 +162,7 @@ pub fn get_settings() -> AppSettings {
     settings_store().read().expect("读取设置锁失败").clone()
 }
 
-pub fn update_settings(mut new_settings: AppSettings) -> Result<(), String> {
+pub fn update_settings(mut new_settings: AppSettings) -> Result<(), AppError> {
     new_settings.normalize_paths();
     new_settings.save()?;
 
@@ -183,4 +185,4 @@ pub fn get_codex_override_dir() -> Option<PathBuf> {
         .codex_config_dir
         .as_ref()
         .map(|p| resolve_override_path(p))
-}
+}