Desktop app: keyboard shortcut to select working directory (keyboard-only workflow)

What

Add a keyboard shortcut to select/change the session working directory in the Claude Desktop app, so the flow can be completed without a mouse.

Why

The Desktop app has a mouse-driven Project folder picker for choosing the working directory, but there is no keyboard path to it. The CLI slash commands /cd and /add-dir are not available in Desktop (/cd returns "not available in this environment"), so a keyboard-only user currently has no way to change the working directory at all — the mouse is the only option.

Proposed behavior

1. Cmd+N starts a new session (already works today).
2. Add a keyboard shortcut that opens/focuses the Project folder picker so a directory can be selected and confirmed entirely from the keyboard.

Ideally the picker itself is also fully keyboard-navigable (arrow keys + Enter) once opened.

Notes

This is distinct from the existing directory-picker requests (e.g. #54461, #36175), which concern having a picker or switching cwd at runtime. The gap here is specifically keyboard accessibility of directory selection for keyboard-driven workflows.