feat(settings): add enableLocalProxy toggle to gate main page proxy UI

New users often accidentally trigger ProxyToggle/FailoverToggle on the
main page. Add a settings toggle (default off) so the proxy controls
only appear when explicitly enabled. The proxy service start/stop in
settings remains independent of this visibility flag.

src/components/proxy/ProxyPanel.tsx

@@ -408,9 +408,6 @@ export function ProxyPanel() {
           </div>
         ) : (
           <div className="space-y-6">
-            {/* 空白区域避免冲突 */}
-            <div className="h-4"></div>
-
             {/* 基础设置 - 监听地址/端口 */}
             <div className="rounded-lg border border-border bg-muted/40 p-4 space-y-4">
               <div>