fix: add Gemini 3.x before 2.5 retirement

[CorieW]

Summary

• Add Gemini 3.6 Flash, 3.5 Flash, 3.5 Flash Lite, 3.1 Flash Lite, and 3.1 Pro Preview to firestore-translate-text and default to 3.6 Flash. Gemini 2.5 options remain until they retire in October 2026.
• Switch storage-resize-images content filter from gemini-2.5-flash to gemini-3.1-flash-lite.
• Bump firestore-translate-text to 0.1.31 and storage-resize-images to 0.3.7, with CHANGELOG and generated README updates.

Why gemini-3.1-flash-lite (not gemini-3.5-flash-lite or higher)

The resize content filter is a high-volume yes/no image check, previously hardcoded to gemini-2.5-flash. We picked gemini-3.1-flash-lite because it is similar in price to that previous default ($0.25/$1.50 vs $0.30/$2.50 per 1M tokens) and performs better (Google: similar or better quality than 2.5 Flash, ~2.5× faster time-to-first-token). gemini-3.5-flash-lite is the same sticker as 2.5 Flash; gemini-3.5-flash and gemini-3.6-flash are about 5× input.

Test plan

• Confirm translate installer lists 3.x models and defaults to gemini-3.6-flash
• Confirm 2.5 models still selectable for existing installs
• Run storage-resize-images content-filter unit tests
• Smoke a Gemini translation with the new default model

[gemini-code-assist]

Code Review

This pull request updates the Gemini models used in the firestore-translate-text and storage-resize-images extensions. Specifically, it introduces Gemini 3.6 Flash, 3.1 Flash Lite, and 3.1 Pro Preview model options, setting Gemini 3.6 Flash as the new default model. It also updates the content filter in the image resizing extension to use Gemini 3.6 Flash, and updates the corresponding documentation, configuration files, and tests. There are no review comments, so I have no feedback to provide.