feat(settings): add upstream network proxy modes

[GcsSloop]

Summary

• add upstream network proxy settings with system/direct/manual modes
• route upstream official, ppchat, lua and gateway HTTP traffic through the dynamic proxy client
• expose the new settings in the settings page and validate them with smoke tests against local proxy 127.0.0.1:7897

Verification

• cd backend && go test ./...
• cd frontend && pnpm test -- --run src/features/settings/SettingsPage.test.tsx
• cd frontend && pnpm build
• local smoke: system mode fails with invalid proxy env, succeeds with 127.0.0.1:7897, manual mode succeeds with 127.0.0.1:7897