Term from glossary with bilingual format always shown as untranslateable in source language

### Describe the issue

Source language terms in a glossary with the TBX format always inherit the read-only flag because "_editing source strings is not available for bilingual formats._"

The `read-only` flag is also used on the unit itself for marking as untranslatable.

When translating a component in the glossaries source language, the snippet in the translate view renders all terms with the "This term should not be translated." tooltip text, regardless if they are marked as untranslatable. This is confusing.

### I already tried

- [x] I've read and searched [the documentation](https://docs.weblate.org/).
- [x] I've searched for similar filed issues in this repository.

### Steps to reproduce the behavior

1. Create a glossary with TBX format
2. Add a term to the glossaries source language
3. Use the glossary in a component in the same language
4. Term is always displayed as should not be translated, which is confusing

### Expected behavior

Only terms that are actually marked as untranslatable should show the tooltip.

### Screenshots

_No response_

### Exception traceback

```pytb

```

### How do you run Weblate?

Docker container

### Weblate version

2026.8.1

### Weblate deploy checks

```shell

```

### Additional context

- #15680

Reproduce the issue with a TBX glossary, a source-language term, and a component using that glossary, then inspect how the translate view renders the term. Compare the read-only flag inherited from the bilingual format with the unit's untranslatable state; done means only genuinely untranslatable terms show the "This term should not be translated." tooltip.

Written by the indexing model from the issue text.