fix(desktop): preserve curated provider model lists

[closss]

Summary

This fixes the desktop provider model refresh flow so fetched model lists no longer overwrite a user's curated model list.

Before this change:

• Background model discovery on the Settings usage tab replaced saved provider models with the full API response.
• Saving an API key and auto-refreshing could also replace the saved list.
• Manual refresh replaced the list instead of preserving existing curated entries.

After this change:

• Background and API-key auto-refresh preserve a non-empty saved model list, and only populate providers that do not have models yet.
• Manual refresh appends newly discovered models while keeping existing entries intact.
• The provider default model is preserved when still available, otherwise it falls back to the first saved model.

Issues

Fixes #3465
Fixes #3506
Fixes #3529

Implementation notes

• Extracted provider model merge/default selection logic into desktop/frontend/src/lib/providerModels.ts.
• Reused that helper in the three model-refresh paths in SettingsPanel.tsx.
• Added focused tests for manual merge behavior, background curation preservation, empty-list population, and default fallback.
• Kept the patch scoped to desktop settings model-refresh behavior.

Tests

• npm run test:all
• git diff --check
• gofmt -l
• go test ./...

[esengine]

Full CI matrix is green after the re-trigger, and the model-list merge logic reviewed cleanly (preserve-curated vs union merge, with a focused test). Approving.