fix: use joplin.clipboard API for copy

[PierreLga456]

This PR replaces the deprecated navigator.clipboard call
with Joplin's native joplin.clipboard API.

✅ Works on macOS (Sequoia/Tahoe) and Linux (Mint Cinnamon)

[PierreLga456]

🧾 Pull Request Description (ready-to-copy)

Summary
This PR fixes the clipboard copy feature by replacing deprecated browser clipboard access with Joplin’s official API.

Details
• Replaces navigator.clipboard.writeText() with await joplin.clipboard.writeText()
• Removes all joplin.views.dialogs.showMessageBox() calls that caused blocking issues (notably on Linux GTK)
• Keeps user feedback handled directly by the Outline panel, avoiding redundant popups
• Improves UX consistency and reliability across platforms

Tested on
• ✅ macOS 15.0 “Tahoe” (Apple Silicon)
• ✅ Linux Mint Cinnamon (T2 hardware)
• No regressions, no crashes, clipboard works instantly

Why it matters
The previous implementation triggered modal dialogs that could freeze the UI on Linux and interrupt workflow.
This patch ensures seamless copy operations using Joplin’s own clipboard API, aligning with plugin API best practices.

[cqroot]

Hi, thank you for your contribution. I've just released version 1.5.15, and this PR #108 should have fixed your issue.