Introduce a single, app-root-owned source of truth for fullscreen state,
mirroring the existing provider pattern (ToastProvider, FileDropProvider).
The provider centralizes the fullscreenchange / win-visibility-changed /
keydown listeners and exposes the same [fullscreen, requestFullscreen,
exitFullscreen, toggleFullscreen] tuple that consumers already destructure.
Not yet wired up - both the legacy src/common/useFullscreen hook and the
new module coexist. Subsequent commits mount the provider in App.js and
switch consumers over.
Made-with: Cursor
