Add more output formats and high-density rendering

[mintyPT]

Problem

ClickClick currently outputs PNG and JPEG only. A world-class image CLI should support modern web formats, print-friendly formats where reasonable, and high-density output.

Proposal

Add additional output formats and scale controls.

Acceptance criteria

• Add WebP output support if Chromium or a post-processing path can provide it reliably.
• Evaluate AVIF support and document the decision.
• Evaluate PDF export for print and poster workflows.
• Add --scale or device scale factor support for retina outputs.
• Ensure output format inference works from file extensions.
• Add validation for incompatible options.
• Add tests for supported formats and scale behavior.
• Update README examples and package verification as needed.