fix: shrink file tree code blocks

[luozejian]

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Fixes #258

Problem

File tree code blocks in documentation and blog posts currently expand to the full content width, even when the tree itself is much narrower. This leaves a large amount of empty space and makes file trees look disproportionate compared with their content.

Solution

Shrink tree code blocks to their intrinsic content width on non-mobile viewports while keeping them capped at the available content width.

The inner pre uses automatic width so it can contribute its intrinsic size, with extra right padding reserved for the copy button. Mobile layouts keep the existing full-width block and horizontal scrolling behavior.

This only targets code blocks whose generated pre has data-language="tree"; other code blocks are unchanged.

Validation

Chrome layout measurements:

• Docs project structure page: 749px block reduced to 354px
• Blog file trees: 856px blocks reduced to approximately 245px and 254px
• 320px mobile viewport: block remains 288px wide with internal horizontal scrolling

The following checks pass:

• pnpm --filter @sveltejs/site-kit lint
• pnpm --filter @sveltejs/site-kit check
• pnpm --filter svelte.dev check
• pnpm build

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