fix: address code review findings for DNS management

- CRITICAL: Change DNS zones endpoint from GET to POST to avoid
  leaking API token in URL query parameters
- HIGH: Add sync.RWMutex to protect dnsProvider field in Server,
  Deployer, and proxy Manager against concurrent read/write races
- HIGH: Capture old DNS provider reference synchronously before
  launching background cleanup goroutine
- HIGH: Use getDNS()/getDNSProviderLocked() accessors instead of
  direct field reads in all DNS operations

web/.svelte-kit/output/server/chunks/IconLoader.js

@@ -0,0 +1,8 @@
+import { a as attr, b as attr_class, g as stringify } from "./index.js";
+function IconLoader($$renderer, $$props) {
+  const { size = 20, class: c = "" } = $$props;
+  $$renderer.push(`<svg xmlns="http://www.w3.org/2000/svg"${attr("width", size)}${attr("height", size)} viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"${attr_class(`animate-spin ${stringify(c)}`)} aria-hidden="true"><path d="M21 12a9 9 0 1 1-6.219-8.56"></path></svg>`);
+}
+export {
+  IconLoader as I
+};