fix(#424): login screen buttons unresponsive after logout (#427)

Replace LaunchedEffect(startScreen) pattern with remember(startScreen)
keyed recreation of NavBackStack. Mutating the same NavBackStack by
clearing and re-adding entries confuses Navigation3's internal state,
causing duplicate screen entries that compete for touch events.

The fix creates a fresh NavBackStack whenever startScreen changes
(chat -> landing), avoiding any stale-state mutation issues.

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/java/com/m57/hermescontrol/Navigation.kt

@@ -111,13 +111,7 @@ fun MainNavigation(sessionId: String? = null) {
     val hasToken = !token.isNullOrBlank()
     val startScreen: NavKey = if (hasToken) ChatScreen else LandingScreen
 
-    val backStack = remember { NavBackStack(startScreen) }
-    LaunchedEffect(startScreen) {
-        if (backStack.lastOrNull() != startScreen) {
-            backStack.clear()
-            backStack.add(startScreen)
-        }
-    }
+    val backStack = remember(startScreen) { NavBackStack(startScreen) }
     NavigationController.backStack = backStack
 
     val currentScreen = backStack.lastOrNull() ?: startScreen