fix(failover): switch to P1 immediately when enabling auto failover

Previously, enabling auto failover kept using the current provider until
the first failure, causing inconsistency when the current provider was
not in the failover queue. When stopping proxy, the restored config
would not match user expectations.

New behavior:
- Enable auto failover = immediately switch to queue P1
- Subsequent routing follows queue order (P1→P2→...)
- Auto-add current provider to queue if queue is empty

Changes:
- Add switch_proxy_target() for hot-switching during proxy mode
- Update provider_router to use queue order when failover enabled
- Sync tray menu Auto click with the same logic
- Update UI tooltips to reflect new semantics
- Add tests for queue-only routing scenario

src/components/proxy/FailoverToggle.tsx

@@ -39,11 +39,11 @@ export function FailoverToggle({ className, activeApp }: FailoverToggleProps) {
   const tooltipText = isEnabled
     ? t("failover.tooltip.enabled", {
         app: appLabel,
-        defaultValue: `${appLabel} 故障转移已启用\n自动切换到下一个可用供应商`,
+        defaultValue: `${appLabel} 故障转移已启用\n按队列优先级（P1→P2→...）选择供应商`,
       })
     : t("failover.tooltip.disabled", {
         app: appLabel,
-        defaultValue: `启用 ${appLabel} 故障转移\n当当前供应商失败时自动切换`,
+        defaultValue: `启用 ${appLabel} 故障转移\n将立即切换到队列 P1，并在失败时自动切换到下一个`,
       });
 
   return (