fix(settings): list built-in provider default models

[fly1d]

Summary

• add the existing Gemini, Mistral, and NVIDIA default model IDs to their curated Settings selectors
• prevent untouched built-in configurations from being presented as Custom... with an unnecessary text field
• add a Chrome/Firefox regression that checks all 17 curated core providers against their runtime defaults

Testing

• node test/run.js (1807 passed, 2 failed; both failures are present on current main: the missing dist/webbrain-chrome-32.1.0.zip artifact and the stale Apocalypse Wikipedia callout visibility assertion)
• npm run test:toolbar-guard (33 passed)
• npm run test:security (60/60 passed)
• node --check for all modified JavaScript files
• git diff --check

Partially addresses #2816.

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