fix: prevent code blocks from overflowing post layout

[Heersin]

Motivation

• Long fenced code blocks in article content could stretch the post layout because the theme did not constrain pre/code elements, so add styles to keep code inside the article container and enable scrolling.

Description

• Add .prose pre rules to set max-width: 100%, overflow-x: auto, padding, border and a dark background to make long code lines scrollable inside the post.
• Add .prose code font-family and font-size for consistent monospace rendering inside prose.
• Add .prose pre code { white-space: pre; } to ensure fenced code preserves original whitespace and line breaks.

Testing

• Confirmed the stylesheet assets/css/main.css was updated and the diff contains the new .prose pre/.prose code rules.
• Attempted to run hugo server -s exampleSite -t bitblog-theme, which failed because hugo is not installed in this environment.
• Attempted a Playwright screenshot of the local site, which failed because the dev server was not reachable, so no rendered screenshot was produced.

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Codex Task