fix: handle missing realtime audio defaults

[nkgotcode]

Pull request checklist

• The PR has a proper title. Use Semantic Commit Messages. (No more branch-name title please)

• Make sure this is ready to be merged into the relevant branch. Please don't create a PR and let it hang for a few days.

• Ensure you can run the codes you submitted succesfully. These submissions will be prioritized for review:

  Introduce improvements in program execution speed;

  Introduce improvements in synthesis quality;

  Fix existing bugs reported by user feedback (or you met);

  Introduce more convenient user operations.

PR type

• Bug fix

Description

• Fixes realtime GUI startup when the saved/default sounddevice ids are missing from the currently available input/output device lists.
• The realtime GUI now falls back to the first available filtered input/output device instead of failing while loading config.

Validation:

• python3 -m py_compile gui_v1.py
• Targeted realtime config regression for sd.default.device = [None, 99]

Screenshot

• Not applicable; startup fallback behavior only.