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

.claude/settings.json

@@ -2,7 +2,19 @@
   "permissions": {
     "allow": [
       "Bash(npm install:*)",
-      "Bash(go build:*)"
+      "Bash(go build:*)",
+      "Bash(npx svelte-check:*)",
+      "Bash(curl -s -o /dev/null -w \"%{http_code}\" http://localhost:8090/api/settings)",
+      "Bash(npm run:*)",
+      "Bash(curl -s -o /dev/null -w \"%{http_code}\" http://localhost:8090/)",
+      "Bash(go vet:*)",
+      "Bash(git checkout:*)",
+      "Bash(git stash:*)",
+      "Bash(echo \"EXIT: $?\")"
+    ],
+    "additionalDirectories": [
+      "C:\\Users\\Alexei\\Documents\\docker-watcher\\internal",
+      "C:\\Users\\Alexei\\Documents\\docker-watcher\\web\\src\\routes\\projects\\[id]\\volumes\\[volId]"
     ]
   }
 }