feat(settings): add image generation toggle for OpenCode users

[Daltonganger]

Summary

• add a simple default-on image_generation_enabled dashboard setting
• enforce the toggle across Responses HTTP and websocket request paths
• expose a single settings toggle with copy explaining that OpenCode users can turn image generation off if needed

Relationship

• Depends on fix(proxy): prefer budget-safe routing and support image-generation compatibility ("code":"invalid_request_error","param":"tools") #421
• Derived from and linked to Daltonganger/codex-lb#4
• Please review this as follow-up work intended to land on top of fix(proxy): prefer budget-safe routing and support image-generation compatibility ("code":"invalid_request_error","param":"tools") #421's image-generation compatibility changes

Verification

• uv run pytest tests/integration/test_settings_api.py tests/integration/test_openai_compat_features.py tests/integration/test_proxy_responses.py tests/integration/test_http_responses_bridge.py tests/integration/test_auth_middleware.py
• npm run typecheck
• npm run test -- src/features/settings/components/routing-settings.test.tsx src/features/settings/components/totp-settings.test.tsx