fix(i18n): make the language picker actually apply on device

The in-app language picker silently did nothing: AppCompatDelegate.set
ApplicationLocales only syncs to the system from an AppCompatActivity, but
MainActivity was a plain ComponentActivity (with a platform theme). Switch
MainActivity to AppCompatActivity and base Theme.Calendula on
Theme.AppCompat.DayNight.NoActionBar.

Changing the locale recreates the activity; set android:windowBackground to a
DayNight colour matching the Compose background (light #FBFCFE / dark #101316)
so the recreation no longer flashes a contrasting backdrop.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/main/java/de/jeanlucmakiola/calendula/MainActivity.kt

@@ -3,9 +3,9 @@ package de.jeanlucmakiola.calendula
 import android.content.Context
 import android.content.Intent
 import android.os.Bundle
-import androidx.activity.ComponentActivity
 import androidx.activity.compose.setContent
 import androidx.activity.enableEdgeToEdge
+import androidx.appcompat.app.AppCompatActivity
 import androidx.compose.foundation.isSystemInDarkTheme
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.runtime.getValue
@@ -24,7 +24,7 @@ import de.jeanlucmakiola.calendula.ui.theme.CalendulaTheme
 import kotlinx.datetime.LocalDate
 
 @AndroidEntryPoint
-class MainActivity : ComponentActivity() {
+class MainActivity : AppCompatActivity() {
 
     // The occurrence a reminder notification was tapped for (eventId, begin,
     // end — the detail screen's key shape). singleTop + onNewIntent route a

app/src/main/res/values-night/colors.xml

@@ -0,0 +1,6 @@
+<resources>
+    <!-- Dark-scheme window backdrop, matching the Compose dark background/surface
+         (#101316) so activity recreation (e.g. language switch) doesn't flash a
+         lighter grey. See values/colors.xml. -->
+    <color name="window_background">#FF101316</color>
+</resources>

app/src/main/res/values/colors.xml

@@ -3,4 +3,8 @@
     <color name="seed">#FF5C6B7A</color>
     <!-- Adaptive icon background -->
     <color name="ic_launcher_background">#FF5C6B7A</color>
+    <!-- Window backdrop shown during activity recreation (e.g. on a language
+         switch). Matches the Compose light scheme background/surface so the
+         recreation is seamless; overridden for dark in values-night. -->
+    <color name="window_background">#FFFBFCFE</color>
 </resources>

app/src/main/res/values/themes.xml

@@ -1,5 +1,10 @@
 <resources xmlns:tools="http://schemas.android.com/tools">
-    <style name="Theme.Calendula" parent="android:Theme.Material.Light.NoActionBar">
+    <!-- AppCompat (DayNight) parent so MainActivity can be an AppCompatActivity,
+         which is required for AppCompatDelegate.setApplicationLocales (the in-app
+         language picker) to sync to the system. Actual colours are driven by the
+         Compose theme; this is essentially the launch/backdrop theme. -->
+    <style name="Theme.Calendula" parent="Theme.AppCompat.DayNight.NoActionBar">
+        <item name="android:windowBackground">@color/window_background</item>
         <item name="android:statusBarColor">@android:color/transparent</item>
         <item name="android:navigationBarColor">@android:color/transparent</item>
         <item name="android:windowLightStatusBar">true</item>