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/index-server.js

@@ -0,0 +1,9 @@
+import { d as ssr_context } from "./index.js";
+import "clsx";
+function onDestroy(fn) {
+  /** @type {SSRContext} */
+  ssr_context.r.on_destroy(fn);
+}
+export {
+  onDestroy as o
+};