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/i18n/locales/en.json

@@ -864,7 +864,7 @@
       "toggleFailed": "Failed to update status"
     },
     "autoFailover": {
-      "info": "When multiple proxy targets are enabled, the system will try them in priority order. When a provider fails consecutively reaching the threshold, the circuit breaker will open and skip that provider.",
+      "info": "When the failover queue has multiple providers, the system will try them in priority order when requests fail. When a provider reaches the consecutive failure threshold, the circuit breaker will open and skip it temporarily.",
       "configSaved": "Auto failover config saved",
       "configSaveFailed": "Failed to save",
       "retrySettings": "Retry & Timeout Settings",

src/i18n/locales/zh.json

@@ -864,7 +864,7 @@
       "toggleFailed": "状态更新失败"
     },
     "autoFailover": {
-      "info": "当启用多个代理目标时，系统会按优先级顺序依次尝试。当某个供应商连续失败达到阈值时，熔断器会自动打开，跳过该供应商。",
+      "info": "当故障转移队列中配置了多个供应商时，系统会在请求失败时按优先级顺序依次尝试。当某个供应商连续失败达到阈值时，熔断器会打开并在一段时间内跳过该供应商。",
       "configSaved": "自动故障转移配置已保存",
       "configSaveFailed": "保存失败",
       "retrySettings": "重试与超时设置",