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