Fix back-press crash in single-category nav graphs

[Rimusolamus]

Fixes #420.

The onRootScreen check at ShowkaseBrowserApp.kt:459 runs inside NavHost(builder = { ... }), which executes once at graph construction time — before the NavController is attached to its graph. currentDestination is null, startDestinationId is 0, so the check always returns false. In single-category mode (consumers with only @Preview) that false propagates down to the *Groups BackHandler, which then tries to navigate("SHOWKASE_CATEGORIES") — a route that doesn't exist in the single-category graph, hence the crash.

Switched to deriving onRootScreen from the NavBackStackEntry that composable(route) { backStackEntry -> ... } already provides per destination. At that scope the graph is attached and backStackEntry.destination.parent?.startDestinationRoute is the route NavHost was started with. Dropped the onRootScreen: Boolean plumbing param from the 5 private graph builder funcs; screen-level composable signatures are unchanged.

Verified on a consumer app with only @Preview (single-category mode): open browser, drill into a group, press back — no crash, the existing Log.e("BackPressed", "onRootScreen") fires confirming the right branch is taken.