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/provider.rs

@@ -36,10 +36,6 @@ pub struct Provider {
     #[serde(skip_serializing_if = "Option::is_none")]
     #[serde(rename = "iconColor")]
     pub icon_color: Option<String>,
-    /// 是否为代理目标（数据库专用字段，不写入配置文件）
-    #[serde(skip_serializing_if = "Option::is_none")]
-    #[serde(rename = "isProxyTarget")]
-    pub is_proxy_target: Option<bool>,
 }
 
 impl Provider {
@@ -62,7 +58,6 @@ impl Provider {
             meta: None,
             icon: None,
             icon_color: None,
-            is_proxy_target: None,
         }
     }
 }