Split from PR #2752 (part 2 of 3).
Backend:
- /api/models/catalog endpoint for browsing remote model catalogs
- /api/models/fetch endpoint for fetching available models from providers
- Credential reuse with provider/API base matching for security
- Default API base resolution for providers without explicit base
Frontend:
- FetchModelsDialog for importing models from remote providers
- CatalogDialog for browsing and importing from model catalogs
- Static import for FetchModelsDialog (replaces dynamic import from PR1)
- Dynamic import retained for TestModelDialog (PR3 territory)
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