feat(settings): add image generation toggle button for PR #421

[Daltonganger]

Summary

• add the settings toggle button for image generation in the Routing settings UI
• keep this PR limited to the button-only UI addition
• link this button addition to the image-generation compatibility work in fix(proxy): prefer budget-safe routing and support image-generation compatibility ("code":"invalid_request_error","param":"tools") #421

Important dependency

This PR should only be merged after #421 is accepted and merged.

This change is intended as a follow-up UI addition for #421:

• fix(proxy): prefer budget-safe routing and support image-generation compatibility ("code":"invalid_request_error","param":"tools") #421

Without #421, this button does not belong on its own.

Verification

• npm run test -- src/features/settings/components/routing-settings.test.tsx