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}
                 />
               )}