Implement button to copy output to clipboard

[lukasaldersley]

• Implemented Copy-To-Clipboard using the clipboard API (only available on https sites or localhost)
• fixed incorrect id (exportOption2 vs listOption)
• fixed typo 'ouput' to 'output'
• changed var to let to harden against accidental scope leaks

(this is independent of #4 or #6 and neither requires the other. If you want me to rebase this onto either #4 or #6 instead of merging this, I'd be happy to do so)

[Copilot]

Pull Request Overview

This pull request implements a copy-to-clipboard feature for the markdown output, addressing several minor bugs and code quality improvements along the way.

• Adds a new copy button using the clipboard API for copying output to clipboard
• Fixes HTML element ID mismatches and typos in element names
• Updates variable declarations from var to let for better scoping

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
script.js	Adds copyExport function, fixes variable declarations, corrects element ID references
index.html	Adds copy button, fixes label-input ID mismatch, corrects typos in element IDs

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[farnots]

Thanks for your PR @lukasaldersley. I made changes on #8 and merge it. Sorry for the duplicate