feat(settings): add per-adapter default model setting

[MattPua]

Summary

Adds a default model preference per adapter (Claude / Codex), mirroring the existing default reasoning effort setting.

• Each adapter remembers its own default independently, so switching adapters no longer silently drops a stored default belonging to the other
• settingsStore: defaultModels keyed by adapter ("last_used" fallback per adapter)
• usePreviewConfig: applies the per-adapter default when restoring the model on a new task
• GeneralSettings: two rows — Claude always visible, Codex rendered only when models are available
• useAvailableModelOptions: shared fetch + flatten hook so settings and task input agree on the visible model list

Test plan

• Open Settings → confirm both Default Claude model and Default Codex model rows render
• Set a Claude default, start a new Claude task → model is pre-selected
• Switch to Codex, start a new task → Claude default is not applied; Codex uses last-used or its own default
• Set a Codex default, switch back to Claude → Claude default still respected

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Created with PostHog Code

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[github-actions]

React Doctor found no issues in the changed files. 🎉

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