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/src/commands/mcp.rs

@@ -58,7 +58,7 @@ pub async fn get_mcp_config(
         .to_string();
     let mut cfg = state
         .config
-        .lock()
+        .write()
         .map_err(|e| format!("获取锁失败: {}", e))?;
     let app_ty = AppType::from(app.as_deref().unwrap_or("claude"));
     let (servers, normalized) = mcp::get_servers_snapshot_for(&mut cfg, &app_ty);
@@ -84,7 +84,7 @@ pub async fn upsert_mcp_server_in_config(
 ) -> Result<bool, String> {
     let mut cfg = state
         .config
-        .lock()
+        .write()
         .map_err(|e| format!("获取锁失败: {}", e))?;
     let app_ty = AppType::from(app.as_deref().unwrap_or("claude"));
     let mut sync_targets: Vec<AppType> = Vec::new();
@@ -115,7 +115,7 @@ pub async fn upsert_mcp_server_in_config(
 
     let cfg2 = state
         .config
-        .lock()
+        .read()
         .map_err(|e| format!("获取锁失败: {}", e))?;
     for app_ty_to_sync in sync_targets {
         match app_ty_to_sync {
@@ -135,7 +135,7 @@ pub async fn delete_mcp_server_in_config(
 ) -> Result<bool, String> {
     let mut cfg = state
         .config
-        .lock()
+        .write()
         .map_err(|e| format!("获取锁失败: {}", e))?;
     let app_ty = AppType::from(app.as_deref().unwrap_or("claude"));
     let existed = mcp::delete_in_config_for(&mut cfg, &app_ty, &id)?;
@@ -143,7 +143,7 @@ pub async fn delete_mcp_server_in_config(
     state.save()?;
     let cfg2 = state
         .config
-        .lock()
+        .read()
         .map_err(|e| format!("获取锁失败: {}", e))?;
     match app_ty {
         AppType::Claude => mcp::sync_enabled_to_claude(&cfg2)?,
@@ -169,7 +169,7 @@ pub async fn set_mcp_enabled(
 pub async fn sync_enabled_mcp_to_claude(state: State<'_, AppState>) -> Result<bool, String> {
     let mut cfg = state
         .config
-        .lock()
+        .write()
         .map_err(|e| format!("获取锁失败: {}", e))?;
     let normalized = mcp::normalize_servers_for(&mut cfg, &AppType::Claude);
     mcp::sync_enabled_to_claude(&cfg)?;
@@ -186,7 +186,7 @@ pub async fn sync_enabled_mcp_to_claude(state: State<'_, AppState>) -> Result<bo
 pub async fn sync_enabled_mcp_to_codex(state: State<'_, AppState>) -> Result<bool, String> {
     let mut cfg = state
         .config
-        .lock()
+        .write()
         .map_err(|e| format!("获取锁失败: {}", e))?;
     let normalized = mcp::normalize_servers_for(&mut cfg, &AppType::Codex);
     mcp::sync_enabled_to_codex(&cfg)?;
@@ -216,7 +216,7 @@ fn set_mcp_enabled_internal(
     id: &str,
     enabled: bool,
 ) -> Result<bool, AppError> {
-    let mut cfg = state.config.lock()?;
+    let mut cfg = state.config.write()?;
     let changed = mcp::set_enabled_and_sync_for(&mut cfg, &app_ty, id, enabled)?;
     drop(cfg);
     state.save()?;
@@ -234,7 +234,7 @@ pub fn set_mcp_enabled_test_hook(
 }
 
 fn import_mcp_from_claude_internal(state: &AppState) -> Result<usize, AppError> {
-    let mut cfg = state.config.lock()?;
+    let mut cfg = state.config.write()?;
     let changed = mcp::import_from_claude(&mut cfg)?;
     drop(cfg);
     if changed > 0 {
@@ -249,7 +249,7 @@ pub fn import_mcp_from_claude_test_hook(state: &AppState) -> Result<usize, AppEr
 }
 
 fn import_mcp_from_codex_internal(state: &AppState) -> Result<usize, AppError> {
-    let mut cfg = state.config.lock()?;
+    let mut cfg = state.config.write()?;
     let changed = mcp::import_from_codex(&mut cfg)?;
     drop(cfg);
     if changed > 0 {