feat: add local Unsplash image insertion

[GeekMai90]

Summary

• add Unsplash image selection from the slash menu and editor toolbar, with local download, crop, and insertion into assets/images
• add optional AI article recommendations, Chinese keyword translation, random-photo fallback, and resilient search loading
• add Unsplash and translation service settings plus the 16:9 crop experience with drag, zoom, grid, and reset controls

Validation

• npm run check
• Rust unit tests and frontend tests included in the full check