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/environment.js

@@ -0,0 +1,34 @@
+let base = "";
+let assets = base;
+const app_dir = "_app";
+const relative = true;
+const initial = { base, assets };
+function override(paths) {
+  base = paths.base;
+  assets = paths.assets;
+}
+function reset() {
+  base = initial.base;
+  assets = initial.assets;
+}
+function set_assets(path) {
+  assets = initial.assets = path;
+}
+let prerendering = false;
+function set_building() {
+}
+function set_prerendering() {
+  prerendering = true;
+}
+export {
+  set_building as a,
+  set_prerendering as b,
+  base as c,
+  app_dir as d,
+  assets as e,
+  reset as f,
+  override as o,
+  prerendering as p,
+  relative as r,
+  set_assets as s
+};