refactor(proxy): remove is_proxy_target in favor of failover_queue

- Remove `is_proxy_target` field from Provider struct (Rust & TypeScript)
- Remove related DAO methods: get_proxy_target_provider, set_proxy_target
- Remove deprecated Tauri commands: get_proxy_targets, set_proxy_target
- Add `is_available()` method to CircuitBreaker for availability checks
  without consuming HalfOpen probe permits (used in select_providers)
- Keep `allow_request()` for actual request gating with permit tracking
- Update stream_check to use failover_queue instead of is_proxy_target
- Clean up commented-out reset circuit breaker button in ProviderActions
- Remove unused useProxyTargets and useSetProxyTarget hooks

src-tauri/src/services/provider/mod.rs

@@ -214,9 +214,6 @@ impl ProviderService {
             // Update database is_current
             state.db.set_current_provider(app_type.as_str(), id)?;
 
-            // 同时更新 is_proxy_target（代理路由器使用此字段选择供应商）
-            state.db.set_proxy_target_provider(app_type.as_str(), id)?;
-
             // Update local settings for consistency
             crate::settings::set_current_provider(&app_type, Some(id))?;
 
@@ -229,7 +226,7 @@ impl ProviderService {
             .map_err(|e| AppError::Message(format!("更新 Live 备份失败: {e}")))?;
 
             // Note: No Live config write, no MCP sync
-            // The proxy server will route requests to the new provider via is_proxy_target
+            // The proxy server will route requests to the new provider via is_current
             return Ok(());
         }
 
@@ -287,18 +284,6 @@ impl ProviderService {
         Ok(())
     }
 
-    /// Set proxy target provider
-    pub fn set_proxy_target(state: &AppState, app_type: AppType, id: &str) -> Result<(), AppError> {
-        // Check if provider exists
-        let providers = state.db.get_all_providers(app_type.as_str())?;
-        if !providers.contains_key(id) {
-            return Err(AppError::Message(format!("供应商 {id} 不存在")));
-        }
-
-        state.db.set_proxy_target_provider(app_type.as_str(), id)?;
-        Ok(())
-    }
-
     /// Sync current provider to live configuration (re-export)
     pub fn sync_current_to_live(state: &AppState) -> Result<(), AppError> {
         sync_current_to_live(state)