fix(failover): patch P1-P3 reliability gaps surfaced by team review

- Forwarder buffers non-streaming bodies and primes streaming first
  chunk before signaling success, so body timeouts and SSE first-chunk
  failures route through the circuit breaker instead of being recorded
  as success on response-header arrival
- Atomic enable-failover: switch to P1 before persisting the flag, and
  roll back auto-added queue entries when the switch is rejected
  (e.g. official providers)
- Hot-reload circuit breaker config on per-app proxy config change
  instead of waiting for a proxy restart
- FailoverToggle / FailoverQueueManager / AutoFailoverConfigPanel
  require proxy takeover for the active app; the backend command also
  rejects enabling when takeover is off
- ProviderHealthBadge consumes the backend is_healthy flag instead of
  hardcoding the 5-failure threshold

Cleanup:
- impl From<&AppProxyConfig> for CircuitBreakerConfig and use it from
  the command layer
- Collapse three identical TabsContent blocks into a single map

src-tauri/src/proxy/provider_router.rs

@@ -201,6 +201,17 @@ impl ProviderRouter {
         }
     }
 
+    /// 更新指定应用已创建熔断器的配置（热更新）
+    pub async fn update_app_configs(&self, app_type: &str, config: CircuitBreakerConfig) {
+        let prefix = format!("{app_type}:");
+        let breakers = self.circuit_breakers.read().await;
+        for (key, breaker) in breakers.iter() {
+            if key.starts_with(&prefix) {
+                breaker.update_config(config.clone()).await;
+            }
+        }
+    }
+
     /// 获取熔断器状态
     #[allow(dead_code)]
     pub async fn get_circuit_breaker_stats(