Ability to mark translations read-only from UI

### Describe the problem

When doing a partial translation in a regional language, where the localization framework provides a fallback to a base language, there is no way in the UI to mark units not needing a regional translation. So the translator does not know when their translation is completed.

### Solution brainstorm

Units that do not need to be translated in a specific language could be set read-only, so they count as translated.

For single strings: provide the tools button with the option "Mark as read-only" in all languages' translation view.

For bulk editing: add new section "Read-only to set" as a dropdown with options "Do not change" (default), "Set read-only" and "Remove read-only".

### Describe alternatives you have considered

1. Using a file format that supports per-translation flags: would need tooling outside of Weblate to mange this
1. Have something like "don't translate this" state: it might better reflect the intention but adds too much complexity when read-only is already here
1. Implement full per-translation flag support (#12779 & #12815) and use the `read-only` flag: could work for bulk edit, but too cumbersome for single units, needs #1567 and good UI: "_The only tricky issue is how to reasonably present this to the users_." (from https://github.com/WeblateOrg/weblate/pull/13521#issuecomment-2665982375)

### Screenshots

Bulk edit:

Side note: could also needs some clarification in the Bulk Edit UI that flags and labels affects all translations while state and read-only apply only to the selected language.

### Additional context

Related to #2320 but much narrower in scope.