Handle missing server list URL in online menu

[IIPoliII]

Summary

• add a default constant and validation for the server browser URL in GameSettings
• validate loaded and applied settings so the URL is restored when blank or malformed
• guard the online server refresh logic against missing URLs and surface user-friendly errors
• trim nickname input and save updated settings immediately so the player's name is always persisted

Testing

• not run (Unity editor not available in container)

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https://chatgpt.com/codex/tasks/task_e_68ceceab0c2c8325bc658f6d7f654ce9