Add "default project directory" setting for new sessions in desktop app

Problem

When creating a new session in the Claude Code desktop app, there's no way to configure which project directory it defaults to. Users who have a general-purpose scratch directory for ad-hoc tasks and separate project-specific directories must manually switch the project every time they start a new session.

Proposed solution

Add a setting (e.g. defaultProjectDirectory in settings.json or a UI preference) that controls which directory new sessions open in by default. This should:

• Only affect new sessions — existing sessions keep their own working directory
• Be overridable per session (the project selector still works as usual)
• Be optional (current behavior is the fallback)

Use case

I use a scratch directory for general tasks and switch to specific project folders only when working on dedicated apps. Having to manually select the scratch folder for every new session adds unnecessary friction.