feat: add ImageBytes API for in-memory image download

[spiegel-im-spiegel]

Summary

Add a new (*Webinfo).ImageBytes(ctx) API to download image content into memory.

Changes

• Add ImageBytes(ctx) method to webinfo.go
  • validates nil receiver and empty ImageURL
  • fetches image with configured User-Agent
  • returns response body as []byte
  • wraps errors with context and joins close errors
• Add tests in webinfo_test.go
  • nil receiver case
  • empty URL case
  • success case with httptest server
• Update README.md
  • usage example for ImageBytes
  • Public API list
  • behavior note about memory risk for large images

Validation

• task test