refactor(backend): complete phase 1 - full AppError migration (100%)

Finalized the backend error handling refactoring by migrating all remaining
modules to use AppError, eliminating all temporary error conversions.

## Changes

### Fully Migrated Modules

- **mcp.rs** (129 lines changed)
  - Migrated 13 functions from Result<T, String> to Result<T, AppError>
  - Added AppError::McpValidation for domain-specific validation errors
  - Functions: validate_server_spec, validate_mcp_entry, upsert_in_config_for,
    delete_in_config_for, set_enabled_and_sync_for, sync_enabled_to_claude,
    import_from_claude, import_from_codex, sync_enabled_to_codex
  - Removed all temporary error conversions

- **usage_script.rs** (143 lines changed)
  - Migrated 4 functions: execute_usage_script, send_http_request,
    validate_result, validate_single_usage
  - Used AppError::Message for JS runtime errors
  - Used AppError::InvalidInput for script validation errors
  - Improved error construction with ok_or_else (lazy evaluation)

- **lib.rs** (47 lines changed)
  - Migrated create_tray_menu() and switch_provider_internal()
  - Simplified PoisonError handling with AppError::from
  - Added error logging in update_tray_menu()
  - Improved error handling in menu update logic

- **migration.rs** (10 lines changed)
  - Migrated migrate_copies_into_config()
  - Used AppError::io() helper for file operations

- **speedtest.rs** (8 lines changed)
  - Migrated build_client() and test_endpoints()
  - Used AppError::Message for HTTP client errors

- **app_store.rs** (14 lines changed)
  - Migrated set_app_config_dir_to_store() and migrate_app_config_dir_from_settings()
  - Used AppError::Message for Tauri Store errors
  - Used AppError::io() for file system operations

### Fixed Previous Temporary Solutions

- **import_export.rs** (2 lines changed)
  - Removed AppError::Message wrapper for mcp::sync_enabled_to_codex
  - Now directly calls the AppError-returning function (no conversion needed)

- **commands.rs** (6 lines changed)
  - Updated query_provider_usage() and test_api_endpoints()
  - Explicit .to_string() conversion for Tauri command interface

## New Error Types

- **AppError::McpValidation**: Domain-specific error for MCP configuration validation
  - Separates MCP validation errors from generic Config errors
  - Follows domain-driven design principles

## Statistics

- Files changed: 8
- Lines changed: +237/-122 (net +115)
- Compilation: ✅ Success (7.13s, 0 warnings)
- Tests: ✅ 4/4 passed

## Benefits

- **100% Migration**: All modules now use AppError consistently
- **Domain Errors**: Added McpValidation for better error categorization
- **No Temporary Solutions**: Eliminated all AppError::Message conversions for internal calls
- **Performance**: Used ok_or_else for lazy error construction
- **Maintainability**: Removed ~60 instances of .map_err(|e| format!("...", e))
- **Debugging**: Added error logging in critical paths (tray menu updates)

## Phase 1 Complete

Total impact across 3 commits:
- 25 files changed
- +671/-302 lines (net +369)
- 100% of codebase migrated from Result<T, String> to Result<T, AppError>
- 0 compilation warnings
- All tests passing

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

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

src-tauri/src/speedtest.rs

@@ -3,6 +3,8 @@ use reqwest::{Client, Url};
 use serde::Serialize;
 use std::time::{Duration, Instant};
 
+use crate::error::AppError;
+
 const DEFAULT_TIMEOUT_SECS: u64 = 8;
 const MAX_TIMEOUT_SECS: u64 = 30;
 const MIN_TIMEOUT_SECS: u64 = 2;
@@ -15,13 +17,13 @@ pub struct EndpointLatency {
     pub error: Option<String>,
 }
 
-fn build_client(timeout_secs: u64) -> Result<Client, String> {
+fn build_client(timeout_secs: u64) -> Result<Client, AppError> {
     Client::builder()
         .timeout(Duration::from_secs(timeout_secs))
         .redirect(reqwest::redirect::Policy::limited(5))
         .user_agent("cc-switch-speedtest/1.0")
         .build()
-        .map_err(|e| format!("创建 HTTP 客户端失败: {e}"))
+        .map_err(|e| AppError::Message(format!("创建 HTTP 客户端失败: {e}")))
 }
 
 fn sanitize_timeout(timeout_secs: Option<u64>) -> u64 {
@@ -32,7 +34,7 @@ fn sanitize_timeout(timeout_secs: Option<u64>) -> u64 {
 pub async fn test_endpoints(
     urls: Vec<String>,
     timeout_secs: Option<u64>,
-) -> Result<Vec<EndpointLatency>, String> {
+) -> Result<Vec<EndpointLatency>, AppError> {
     if urls.is_empty() {
         return Ok(vec![]);
     }