refactor(backend): phase 5 - optimize concurrency with RwLock and async IO

Replace Mutex with RwLock for AppState.config to enable concurrent reads,
improving performance for tray menu building and query operations that
previously blocked each other unnecessarily.

Key changes:
- Migrate AppState.config from Mutex<MultiAppConfig> to RwLock<MultiAppConfig>
- Distinguish read-only operations (read()) from mutations (write()) across
  all command handlers and service layers
- Offload blocking file I/O in import/export commands to spawn_blocking threads,
  minimizing lock hold time and preventing main thread blocking
- Extract load_config_for_import() to separate I/O logic from state updates
- Update all integration tests to use RwLock semantics

Performance impact:
- Concurrent reads: Multiple threads can now query config simultaneously
  (tray menu, provider list, MCP config)
- Reduced contention: Write locks only acquired during actual mutations
- Non-blocking I/O: Config import/export no longer freezes UI thread

All existing tests pass with new locking semantics.

src-tauri/tests/mcp_commands.rs

@@ -1,4 +1,4 @@
-use std::fs;
+use std::{fs, sync::RwLock};
 
 use serde_json::json;
 
@@ -37,14 +37,14 @@ fn import_default_config_claude_persists_provider() {
     let mut config = MultiAppConfig::default();
     config.ensure_app(&AppType::Claude);
     let state = AppState {
-        config: std::sync::Mutex::new(config),
+        config: RwLock::new(config),
     };
 
     import_default_config_test_hook(&state, AppType::Claude)
         .expect("import default config succeeds");
 
     // 验证内存状态
-    let guard = state.config.lock().expect("lock config");
+    let guard = state.config.read().expect("lock config");
     let manager = guard
         .get_manager(&AppType::Claude)
         .expect("claude manager present");
@@ -71,7 +71,7 @@ fn import_default_config_without_live_file_returns_error() {
     let home = ensure_test_home();
 
     let state = AppState {
-        config: std::sync::Mutex::new(MultiAppConfig::default()),
+        config: RwLock::new(MultiAppConfig::default()),
     };
 
     let err = import_default_config_test_hook(&state, AppType::Claude)
@@ -113,7 +113,7 @@ fn import_mcp_from_claude_creates_config_and_enables_servers() {
     .expect("seed ~/.claude.json");
 
     let state = AppState {
-        config: std::sync::Mutex::new(MultiAppConfig::default()),
+        config: RwLock::new(MultiAppConfig::default()),
     };
 
     let changed =
@@ -123,7 +123,7 @@ fn import_mcp_from_claude_creates_config_and_enables_servers() {
         "import should report inserted or normalized entries"
     );
 
-    let guard = state.config.lock().expect("lock config");
+    let guard = state.config.read().expect("lock config");
     let claude_servers = &guard.mcp.claude.servers;
     let entry = claude_servers
         .get("echo")
@@ -155,7 +155,7 @@ fn import_mcp_from_claude_invalid_json_preserves_state() {
         .expect("seed invalid ~/.claude.json");
 
     let state = AppState {
-        config: std::sync::Mutex::new(MultiAppConfig::default()),
+        config: RwLock::new(MultiAppConfig::default()),
     };
 
     let err =
@@ -197,13 +197,13 @@ fn set_mcp_enabled_for_codex_writes_live_config() {
     );
 
     let state = AppState {
-        config: std::sync::Mutex::new(config),
+        config: RwLock::new(config),
     };
 
     set_mcp_enabled_test_hook(&state, AppType::Codex, "codex-server", true)
         .expect("set enabled should succeed");
 
-    let guard = state.config.lock().expect("lock config");
+    let guard = state.config.read().expect("lock config");
     let entry = guard
         .mcp
         .codex