Address Gemini code review feedback

- Use first() instead of collect() in MainActivity to avoid redundant
  calls when theme changes at runtime (since SettingsViewModel already
  handles that)
- Extract DarkThemeConfig to night mode mapping into extension functions
  to avoid code duplication and improve maintainability

Based on feedback from gemini-code-assist review

app/src/main/kotlin/com/google/samples/apps/nowinandroid/util/DarkThemeConfigExt.kt

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2022 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.samples.apps.nowinandroid.util
+
+import androidx.appcompat.app.AppCompatDelegate
+import com.google.samples.apps.nowinandroid.core.model.data.DarkThemeConfig
+
+/**
+ * Converts [DarkThemeConfig] to the corresponding [AppCompatDelegate] night mode constant.
+ * This is used to set the application-level night mode for the splash screen and system UI.
+ */
+fun DarkThemeConfig.toNightMode(): Int = when (this) {
+    DarkThemeConfig.FOLLOW_SYSTEM -> AppCompatDelegate.MODE_NIGHT_FOLLOW_SYSTEM
+    DarkThemeConfig.LIGHT -> AppCompatDelegate.MODE_NIGHT_NO
+    DarkThemeConfig.DARK -> AppCompatDelegate.MODE_NIGHT_YES
+}

feature/settings/impl/src/main/kotlin/com/google/samples/apps/nowinandroid/feature/settings/impl/DarkThemeConfigExt.kt

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2022 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.samples.apps.nowinandroid.feature.settings.impl
+
+import androidx.appcompat.app.AppCompatDelegate
+import com.google.samples.apps.nowinandroid.core.model.data.DarkThemeConfig
+
+/**
+ * Converts [DarkThemeConfig] to the corresponding [AppCompatDelegate] night mode constant.
+ * This is used to set the application-level night mode for the splash screen and system UI.
+ */
+internal fun DarkThemeConfig.toNightMode(): Int = when (this) {
+    DarkThemeConfig.FOLLOW_SYSTEM -> AppCompatDelegate.MODE_NIGHT_FOLLOW_SYSTEM
+    DarkThemeConfig.LIGHT -> AppCompatDelegate.MODE_NIGHT_NO
+    DarkThemeConfig.DARK -> AppCompatDelegate.MODE_NIGHT_YES
+}