Guard against updates after destroy

[Pappmann]

Summary

Prevent async callbacks and UI updates from touching destroyed widgets after the extension is disabled, which can trigger
“object disposed” errors.

Changes

• Track a _destroyed flag on ClipboardIndicator.
• Early‑return in _buildMenu, _refreshIndicator, and menu helper methods if destroyed.
• Skip post‑build initialization when teardown already happened.

Testing

Manual: reloaded the extension and exercised clipboard history and menu.