frontend: configure Next image remote patterns

[dreamgene]

Summary

• replace the wildcard Next/Image remote host config with an environment-driven allowlist
• add a tested parser for CDN hostnames and full remote pattern URLs
• document the new NEXT_PUBLIC_IMAGE_REMOTE_PATTERNS configuration in .env.example

Testing

• npx vitest run src/lib/image-remote-patterns.test.ts
• npm run build

Notes

• npm run lint is currently blocked by a pre-existing ESLint flat-config plugin redefinition error for jsx-a11y

Closes #701

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