fix(ui): persist environment variable removal (NeuralNomadsAI#581)

## Summary

- Send an explicit JSON Merge Patch deletion when removing an
environment variable.
- Prevent deleted variables from remaining in config.yaml and being
injected into future OpenCode instances.
- Maybe closes NeuralNomadsAI#575

## Root Cause

The previous removal flow submitted the remaining environment-variable
record without the selected key. The settings API recursively applies
JSON Merge Patch, where an omitted nested key means no change rather
than deletion.

## Validation

- npm run typecheck --workspace @codenomad/ui
- npm run build --workspace @codenomad/ui
- git diff --check

Author: pascalandr

packages/ui/src/stores/preferences.tsx

@@ -647,9 +647,9 @@ function addEnvironmentVariable(key: string, value: string, secure: boolean = tr
 }
 
 function removeEnvironmentVariable(key: string): void {
-  const current = serverSettings().environmentVariables
-  const { [key]: removed, ...rest } = current
-  updateEnvironmentVariables(rest)
+  void patchConfigOwner("server", { environmentVariables: { [key]: null } }).catch((error) =>
+    log.error("Failed to remove environment variable", error),
+  )
 }
 
 function isSecureEnvVar(key: string): boolean {