fix: avoid expanding saved image test batches after model renames

[voodoohop]

• Pollinations renamed catalog IDs, keeping old names as aliases. A saved selection such as flux currently matches nothing, then becomes every model; Test All (including autotest) can send unintended paid requests.
• Resolve saved aliases to the matching catalog entries. Keep empty or unavailable saved selections empty, including when catalog loading fails. First-time visitors still get the existing default selection.
• Add seven offline regression tests exercising the actual catalog loader and test runner. Verify old/canonical catalogs, aliases, duplicates, empty/removed/video selections, network fallback, and first-time behavior.
• Run node --test image-comparison/model-selection.test.cjs (7 passing). Inline script syntax also passes. No generation requests were made.