Extract the clipboard write shared by the copy features

[enf0rc3]

Pure extraction, split out of #3303 so the heading work reviews on its own.

copy-markdown.js carries a clipboard write with two code paths: navigator.clipboard.writeText where a secure context allows it, falling back to a hidden textarea plus document.execCommand('copy') for HTTP and older browsers. The heading copy button in #3303 needs exactly the same logic, so it moves to src/scripts/modules/clipboard.js rather than becoming a second copy.

What changed

• writeToClipboard and execCommandCopyFallback move out of the CopyMarkdown class into a module, unchanged apart from the warning prefix ([copy-md] → [clipboard]) and JSDoc types.
• copy-markdown.js imports it and calls writeToClipboard(text) in place of this.writeToClipboard(text).

No behavior change: the same two paths are tried in the same order, and the return contract (true on success) is the same.

code-blocks.js has its own simpler inline version. It is bound up in a larger icon-swapping flow, so it is left alone — worth a follow-up if this module proves out.

Verification

• pnpm build exits 0, 2697 pages, on this branch alone.
• The extracted code is bundled into the page script ([clipboard] appears in dist/docs/_astro/Default.astro_astro_type_script_index_0_lang.*.js), so the import resolves rather than being tree-shaken away.
• prettier --check and cspell on both files: clean.

Stacked on #3301.

🤖 Generated with Claude Code

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[enf0rc3]

Superseded by #3303. navigator.clipboard.writeText has been Baseline since 2020, so the shared module and its execCommand fallback are unnecessary - the four-line write is inlined at the call site instead.