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/commands/proxy.rs

@@ -133,9 +133,17 @@ pub async fn update_proxy_config_for_app(
     config: AppProxyConfig,
 ) -> Result<(), String> {
     let db = &state.db;
+    let app_type = config.app_type.clone();
+    let circuit_config = CircuitBreakerConfig::from(&config);
+
     db.update_proxy_config_for_app(config)
         .await
-        .map_err(|e| e.to_string())
+        .map_err(|e| e.to_string())?;
+
+    state
+        .proxy_service
+        .update_circuit_breaker_config_for_app(&app_type, circuit_config)
+        .await
 }
 
 async fn get_default_cost_multiplier_internal(