fix(app): keep selected model across new sessions

[anduimagui]

Issue for this PR

Closes #4930

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Desktop currently restores model and agent state for existing sessions, but starting a new session in the same project can still reset the selected model back to the agent default. That means a manual model choice is not sticky across later new sessions.

This change stores the app's selected model in workspace-scoped persistence and updates the new-session reset path to only restore agent defaults when there is no current model selection to keep. Existing session restore behavior stays the same, while new sessions in the same project keep the selected model.

Related context: #12582 fixed CLI fallback to recent models, but it did not change the desktop app's new-session behavior.

How did you verify your code works?

I added a unit test covering the new-session reset path so it does not overwrite an existing selected model.

I verified locally with:

• bun test --preload ./happydom.ts ./src/pages/session/session-model-helpers.test.ts
• bun typecheck

Screenshots / recordings

Not included. This changes state persistence behavior rather than visible UI.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Found a potentially related PR:

#14682: fix(tui): retain model and agent selection when switching sessions

This PR appears related as it addresses retaining model and agent selection, but it's specifically for the TUI (text user interface), while PR #17190 is for the desktop app. Worth checking if there's any shared logic or if they're addressing the same underlying issue from different interfaces.

[github-actions]

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