Fix code block overflow and scrollbar on mobile

[simonhamp]

Summary

• Wraps Torchlight code blocks in an overflow-hidden rounded-md container so background and border-radius stay fully in view while only content scrolls horizontally
• Adds min-w-0 to the blog article prose container so code blocks are properly constrained in the flex layout (matching docs and plugin pages)
• Styles the scrollbar with a transparent track and a subtle thumb that only appears on hover, preventing it from occluding the rounded corners
• Moves border-radius and margin styling to CSS (.torchlight-with-copy) so it also applies to pre-rendered plugin README HTML stored in the database

Test plan

• View a docs page with code blocks on mobile — content should scroll horizontally within the code block container, with background and rounded corners fully visible
• View a blog article with code blocks (e.g. /blog/nativephp-v31-the-biggest-performance-leap-yet) — code blocks should not cause the page to widen
• Hover over a scrollable code block — a subtle scrollbar thumb should appear
• Check plugin README pages with code blocks render correctly

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