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/server.rs

@@ -360,6 +360,17 @@ impl ProxyServer {
         self.state.provider_router.update_all_configs(config).await;
     }
 
+    pub async fn update_circuit_breaker_config_for_app(
+        &self,
+        app_type: &str,
+        config: super::circuit_breaker::CircuitBreakerConfig,
+    ) {
+        self.state
+            .provider_router
+            .update_app_configs(app_type, config)
+            .await;
+    }
+
     /// 重置指定 Provider 的熔断器
     pub async fn reset_provider_circuit_breaker(&self, provider_id: &str, app_type: &str) {
         self.state