feat: data foundation (models, secure storage, Retrofit API, repository, L10n)

- java.time via core library desugaring (minSdk 24)
- Domain models: CalEvent, AppSettings, account/calendar models
- CredentialStore (EncryptedSharedPreferences) + SettingsStore
- Retrofit CalendarrApi covering all server endpoints
- CalendarRepository: auth, settings, accounts, events (org.json parsing
  for mixed-type payloads), writable calendars, visibility toggles
- Hilt DI (Moshi), dynamic-baseUrl ApiProvider, AuthInterceptor
- L10n (de/en) ported from iOS

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/main/java/com/scarriffle/calendarr/di/AppModule.kt

@@ -0,0 +1,21 @@
+package com.scarriffle.calendarr.di
+
+import com.squareup.moshi.Moshi
+import com.squareup.moshi.kotlin.reflect.KotlinJsonAdapterFactory
+import dagger.Module
+import dagger.Provides
+import dagger.hilt.InstallIn
+import dagger.hilt.components.SingletonComponent
+import javax.inject.Singleton
+
+@Module
+@InstallIn(SingletonComponent::class)
+object AppModule {
+
+    @Provides
+    @Singleton
+    fun provideMoshi(): Moshi =
+        Moshi.Builder()
+            .add(KotlinJsonAdapterFactory())
+            .build()
+}