Split from PR #2752 (part 1 of 3).
Backend:
- CRUD model endpoints (list/add/update/delete/set-default)
- Provider metadata with default API bases and model provider options
- Model ID validation and normalization
- Anthropic default API base normalization
Frontend:
- Provider registry with metadata, labels, icons, and aliases
- Provider combobox with backend option merging
- Model field validation with provider-aware checks
- Redesigned add/edit model sheets with provider selection
- Dynamic imports for fetch/catalog/test dialogs (coming in PR2/PR3)
- i18n support for model configuration UI
4993cf4a36
