feat(navigation): implement Bottom Navigation Bar with 4 tabs and app shell
MainScreen.kt: new app shell with Scaffold + Material 3 NavigationBar
providing 4 tabs (Uebersicht, Inventur, Warnungen, Einstellungen).
TopLevelDestination.kt: enum defining tab routes, icons (Home, Inventory2,
Warning, Settings), and labels for the navigation bar.
Screen.kt: added Warnings and Settings sealed interface members.
KrisenvorratNavGraph.kt: accepts Modifier, added Warnings/Settings
composables, removed obsolete DashboardScreen navigation callback.
DashboardScreen.kt: removed Scaffold wrapper and onNavigateToItems param,
now uses Column layout (TopAppBar handled inline).
ItemListScreen.kt: removed onDashboardClick param and Dashboard menu entry
(no longer needed with tab navigation).
WarningsScreen.kt, SettingsScreen.kt: placeholder screens for future impl.
MainActivity.kt: delegates to MainScreen instead of NavGraph directly.
Added material-icons-extended dependency for Inventory2 icon.
Closes #33
a4c0dc63b4