Copy source button is always visible but silently does nothing when translation already exists

Problem/Motivation

Confusing UX — translators may click the button repeatedly thinking it is broken or not working.

The button is always visible but silently does nothing when the translation textarea already has a value (if (textarea && !textarea.value) in babel-copy-source.js).

Steps to reproduce

• Open a translation form for a string that already has a translation.
• Observe the "Copy source" button is visible.
• Click the button.
• Nothing happens — the existing translation is not replaced and no message is shown.

Proposed resolution

Either:

• The button is hidden or disabled when all translation fields are already filled
• The button overwrites existing translations