Port image models to dedicated routes

[faetalize]

Summary

• Move supported image-capable models from chat routing to the image model catalog and resolve requests through Gemini, OpenRouter, or the premium edge path.
• Replace Nano Banana daily allowance UI with image-credit messaging.
• Refine the subscription plans layout, including responsive cards, billing tabs, and FAQ presentation.

Verification

• npm run build
• npm run lint
• npm run test
• npx playwright test tests/e2e/subscription/mobile-layout.spec.ts

Closes #211