Prune deprecated Gemini models and tighten backend/frontend helpers

[AzurCrystal]

Summary

• Remove deprecated Gemini image model mappings and defaults from backend model registries and test payload generation.
• Simplify account deletion flow tests to cover the GetByID-first lookup path and verify repository call order.
• Adjust OAuth signup source normalization to accept only supported providers.
• Add fallback pricing for gpt-5.4-nano and update proxy/SQL helpers to ignore close/write errors cleanly.
• Refresh frontend js-cookie to 3.0.8 and rename delivered-file download handlers for clearer intent.

Testing

• Not run (not requested)

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