* feat(web): clarify model availability and status display
- Rename model availability field from configured to available across backend API and frontend usage
- Keep status as reason classification (configured/unconfigured/unreachable) and show unreachable in UI
- Preserve API key preview even when local service is unreachable
- Update backend tests to assert both availability and status semantics
* fix(web): clarify unreachable model status and wording
- Show unreachable status in model cards instead of API key preview when service is down
- Keep API key placeholder preview in model settings whenever an API key is already saved
- Rename model status wording from configured to available across backend, frontend, and i18n
- Update backend model status tests to match renamed status semantics
* style(web): standardize formatting in handleListModels function
* refactor(web): enforce status field as required to follow backend behavior
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