Set protected XLIFF tags to read-only in translation frontend

### Describe the problem

Our reviewers often edit content in protected XLIFF-tags so that variable names are overlocalized. Shortly this resulted in a heavy manual revert action.

### Describe the solution you'd like

I think Weblate should have at least some basic mechanism that the content in protected XLIFF-tags cannot be edited. From a UX perspective the tags should be read only within the editor.

### Describe alternatives you've considered

_No response_

### Screenshots

_No response_

### Additional context

_No response_