Make ImageMagick optional

[aouxwoux]

What changed

• moved magick from Imports to Suggests so package installation does not require Magick++ development headers
• kept the existing ImageMagick-based graph cleanup when magick is installed
• added a base R PNG fallback when it is not installed
• documented the optional dependency and its behavior in both README sources

Why

Azure Synapse and SparkR environments can install summarytools successfully but fail while compiling the magick dependency because Magick++ is unavailable. Graphs are still useful without the optional image post-processing, so installation should not be blocked by that system dependency.

Validation

• git diff --check
• verified NAMESPACE has no hard magick imports
• verified all Magick calls are guarded by requireNamespace()

R CMD check was not available in the workspace because no R runtime is installed.

Fixes #183