P2 Stream D: ThemeSelectionScreen (replaces ThemePickerDialog)

Full-screen theme picker with 11 theme previews matching iOS
ThemeSelectionView. Live preview on tap. Old dialog deleted and
call-sites migrated to the new route.

Tests use the state-logic fallback pattern (plain kotlin.test rather
than runComposeUiTest) because Compose UI testing in commonTest for
this KMP project is flaky — the existing ThemeManager uses mutableStateOf
plus platform-backed ThemeStorage, which doesn't play well with the
recomposer on iosSimulatorArm64. The behavior under test is identical:
helpers in ThemeSelectionScreenState drive the same code paths the
composable invokes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

composeApp/src/commonMain/kotlin/com/tt/honeyDue/App.kt

@@ -30,6 +30,7 @@ import com.tt.honeyDue.ui.screens.TasksScreen
 import com.tt.honeyDue.ui.screens.VerifyEmailScreen
 import com.tt.honeyDue.ui.screens.VerifyResetCodeScreen
 import com.tt.honeyDue.ui.screens.onboarding.OnboardingScreen
+import com.tt.honeyDue.ui.screens.theme.ThemeSelectionScreen
 import com.tt.honeyDue.viewmodel.OnboardingViewModel
 import com.tt.honeyDue.viewmodel.PasswordResetViewModel
 import androidx.lifecycle.viewmodel.compose.viewModel
@@ -706,6 +707,9 @@ fun App(
                     onNavigateToNotificationPreferences = {
                         navController.navigate(NotificationPreferencesRoute)
                     },
+                    onNavigateToThemeSelection = {
+                        navController.navigate(ThemeSelectionRoute)
+                    },
                     onNavigateToUpgrade = {
                         navController.navigate(UpgradeRoute)
                     }
@@ -719,6 +723,14 @@ fun App(
                     }
                 )
             }
+
+            composable<ThemeSelectionRoute> {
+                ThemeSelectionScreen(
+                    onNavigateBack = {
+                        navController.popBackStack()
+                    }
+                )
+            }
         }
         }