[FEATURE] Desktop app: new session should default to the selected session's folder, not the last-used folder

Current behaviour
In the Claude Code desktop app (Code tab), a new session always starts in the most recently used folder, whichever session is selected in the sidebar at the time.

Requested behaviour
When a session is selected and I start a new session, use the selected session's folder as the default. Alternatively, add a setting to choose between "last used folder" and "selected session's folder".

Why
When switching between several projects, I often open a session in project A, look at it, then want a new session in that same project. Right now the new session opens in whichever project I last used, so I have to re-pick the folder every time.

Workaround
Choosing the folder manually in the picker before the first message works, but it's an extra step each time.