Remember the last used model

[Phantom-VK]

Problem

Settings holds four fields. A default model is not one. Every launch starts on the same hardcoded pick, and the user re-selects every time.

Where

• core/settings.py:34 -> Settings dataclass with only default_output_tokens, currency, show_chunk_warnings, onboarding_dismissed

Fix

Add one field: last_model_id: str = "openai:gpt-4o"

On app start, pre-select it. On every model pick, save it.

Scope

One field, one migration-free default, immediately noticeable improvement. No breaking changes.