- Allow users to select text and copy it as Markdown via the context
menu
- Add "Copy as Markdown" under Export menu to copy entire document to
clipboard
Fixes#12983
<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit
* **New Features**
* Added "Copy as Markdown" to the toolbar clipboard More menu for
selected content.
* Added "Copy as Markdown" to the page export menu to copy entire pages
as Markdown.
* **Behavior**
* Export flow now returns success/failure so the UI shows a dedicated
success or error notification for clipboard exports.
* **Localization**
* Added strings for "Copy as Markdown" and "Copied as Markdown".
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
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Co-authored-by: Whitewater <me@waterwater.moe>
Co-authored-by: lawvs <18554747+lawvs@users.noreply.github.com>
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