fix(deployment): resolve shared env vars using main server

Use `$this->mainServer` when resolving environment variable values across
deployment env generation (runtime, buildtime, nixpacks, args, and secrets
hash) so shared server-scoped values are applied consistently.

Also add `server_id` to `SharedEnvironmentVariable::$fillable` and normalize
the Livewire Blade file newline.

resources/views/livewire/shared-variables/server/index.blade.php

@@ -22,4 +22,4 @@
             </div>
         @endforelse
     </div>
-</div>
+</div>