Accept pasted clipboard attachments in Help

[QuickMythril]

Summary

• accept pasted clipboard files in both new-post and reply textareas
• prefer clipboardData.files and fall back to clipboardData.items for screenshot tools that provide no FileList
• retain Help's existing three-attachment limit, media-service routing, size checks, staged QDN publishing, and ordinary text paste behavior

Verification

• npm test (136 passing)
• npm run build
• git diff --check