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/components/providers/ProviderCard.tsx

@@ -364,6 +364,7 @@ export function ProviderCard({
               {isProxyRunning && isInFailoverQueue && health && (
                 <ProviderHealthBadge
                   consecutiveFailures={health.consecutive_failures}
+                  isHealthy={health.is_healthy}
                 />
               )}
 

src/components/providers/ProviderHealthBadge.tsx

@@ -4,6 +4,7 @@ import { useTranslation } from "react-i18next";
 
 interface ProviderHealthBadgeProps {
   consecutiveFailures: number;
+  isHealthy?: boolean;
   className?: string;
 }
 
@@ -13,6 +14,7 @@ interface ProviderHealthBadgeProps {
  */
 export function ProviderHealthBadge({
   consecutiveFailures,
+  isHealthy,
   className,
 }: ProviderHealthBadgeProps) {
   const { t } = useTranslation();
@@ -29,7 +31,7 @@ export function ProviderHealthBadge({
         bgColor: "bg-green-500/10",
         textColor: "text-green-600 dark:text-green-400",
       };
-    } else if (consecutiveFailures < 5) {
+    } else if (isHealthy !== false) {
       return {
         labelKey: "health.degraded",
         labelFallback: "降级",