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/circuit_breaker.rs

@@ -3,6 +3,7 @@
 //! 实现熔断器模式，用于防止向不健康的供应商发送请求
 
 use super::log_codes::cb as log_cb;
+use super::types::AppProxyConfig;
 use serde::{Deserialize, Serialize};
 use std::sync::atomic::{AtomicU32, Ordering};
 use std::sync::Arc;
@@ -47,6 +48,18 @@ pub struct CircuitBreakerConfig {
     pub min_requests: u32,
 }
 
+impl From<&AppProxyConfig> for CircuitBreakerConfig {
+    fn from(config: &AppProxyConfig) -> Self {
+        Self {
+            failure_threshold: config.circuit_failure_threshold,
+            success_threshold: config.circuit_success_threshold,
+            timeout_seconds: config.circuit_timeout_seconds as u64,
+            error_rate_threshold: config.circuit_error_rate_threshold,
+            min_requests: config.circuit_min_requests,
+        }
+    }
+}
+
 impl Default for CircuitBreakerConfig {
     fn default() -> Self {
         Self {