feat: add llm base url env support

[jtracey93]

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Please confirm you have done the following:

• I have searched existing issues and pull requests (including closed ones) to ensure this isn't a duplicate
• I have read CONTRIBUTING.md

Human Written Description

I use Windows foundry local on my machine to provide my local LLMs. However, every time the service starts, it will pick a non-deterministic port to start on, which therefore means I need a way to provide the latest base URL for the custom LLMs every time. This pull request adds support for adding an environment variable which will be used and checked every time the LLM is needed to be called, which will meet the needs of my use case and many others.

Related Issues/Discussions

Fixes #483

Testing

Screenshots/Videos (if applicable)

AI Assistance

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• AI was used (please describe below)

If AI was used:

• Tools used: GitHub Copilot + Claude Opus 4.5
• How extensively: Fairly minor change so fully let it do its thing, but have reviewed and looks good

[cjpais]

@jtracey93 I see you mention this fixes #483, but it seems to do something different than what is requested there?

[jtracey93]

@cjpais Very fair. I must have associated this very loosely in my mind.

However, it does add the ability to set the URL for a custom LLM via an environment variable, which I would find very useful as today using Windows Foundry Local or other services such as ollama, etc.. I'm having to start those manually and then update the settings via the UI each time I launch Handy or restart my PC and its painful

[cjpais]

yeah understood, I will also get this in, I just wanted to clarify