merge: integrate upstream/main

Reconcile fork with upstream/main to resolve branch divergence.

Cherry-picked changes:
- Selector updates for TOP_P, TEMPERATURE, and SUBMIT buttons (9d31911)

Remaining upstream commits merged without file changes as the fork
already contains equivalent implementations developed independently:
- Function calling improvements (FC-001 to FC-004)
- Gemini 3.1 model support
- English localization
- GUI launcher

Thanks to CJackHwang, catsimple, and all upstream contributors.