Move the design contract for choosing the library game folder out of the top
bar and into Settings > Library. The launcher chrome now reserves the right
edge for the kebab/menu controls, while Settings owns the required folder path
and its set/unset states.
The reference mock now persists `gameFolder: string | null` instead of a
boolean, adds an exercisable GameFolderField, and includes matching CSS so the
prototype reflects the documented interaction. This is still design/reference
only; no runtime Tauri settings code changes here.
Test Plan:
- git diff --cached --check
Refs: none
ddidderr
19ae1938f6
