feat: add Atlas Cloud image provider

[binyangzhu000-sudo]

Summary

• add Atlas Cloud Nano Banana Lite as an image model option
• support both text-to-image and multi-image editing through Atlas Cloud's async generation API
• preserve the existing fal.ai models and default behavior

Validation

• git diff --check
• production compilation completed; the repository's existing Set<number>/TypeScript target mismatch stops the later type-check stage in app/page.tsx
• live text-to-image request through /api/generate-character returned an image URL
• live image-edit request using the generated image returned an image URL
• secret scan passed; no dependency or lockfile changes