Add LM Studio URL configuration field

[mpleshivenkov]

Adds a URL input field to configure LM Studio server endpoint.

Primary Change:

• URL text input field for LM Studio server configuration
• Allows users to specify custom LM Studio server URL

Additional Features:

• Check button to validate server connectivity
• Visual feedback with status indicators
• URL validation and error handling

This enables users to connect to LM Studio servers running on different hosts/ports.

[cla-bot[bot]]

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[mpleshivenkov]

@cla-bot check

[cla-bot[bot]]

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[mpleshivenkov]

Fixed code duplication by extracting URL validation into a shared validate_api_url helper function. Both check_server and save_api_url now use the same validation logic.

[benbrandt]

@mpleshivenkov thanks for sending this in! This is definitely something we would like to support, but we want to think through how to handle this for all providers, since all of them allow for overriding the URL.

In the meantime, feel free to add this setting to your settings.json for LM Studio: https://zed.dev/docs/ai/configuration#custom-provider-endpoint