Model selector doesn't accept valid model IDs outside the built-in dropdown list (e.g. claude-opus-4-8)

## Description
The model picker in Buzz Desktop only allows selecting from a fixed dropdown of preset models. There's no way to enter or select a custom/specific model ID that's valid on Anthropic's API but not in that preset list — e.g. `claude-opus-4-8` (Claude Opus 4.8), a currently active model per Anthropic's own model catalog (https://platform.claude.com/docs/en/about-claude/models/model-ids-and-versions).

## Expected behavior
Either the dropdown includes all currently active Anthropic model IDs, or there's a free-text/custom-ID entry option so users can set a specific model version not yet in the preset list.

## Actual behavior
Selection is locked to whatever subset Buzz's dropdown ships with. Attempts to set `claude-opus-4-8` are not possible through the UI — the field only accepts dropdown selection, with no custom-ID input path.

## Steps to reproduce
1. Open agent/session model settings in Buzz Desktop
2. Attempt to select or type `claude-opus-4-8` as the model
3. Observe: not offered in the dropdown, no custom-entry field available

## Additional context
Reported after multiple attempts. Filed on behalf of a Buzz user via their AI agent (Satoshi/Claude) after confirming `claude-opus-4-8` is a valid, active Anthropic model ID.

Start at the agent/session model settings entry point, where the fixed model dropdown is exposed; the issue names no file or test path. Trace how the selected model is passed to Anthropic, then verify that claude-opus-4-8 can be selected through the resulting model-selection flow.

Written by the indexing model from the issue text.