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/build.gradle.kts

@@ -30,6 +30,7 @@ android {
     compileOptions {
         sourceCompatibility = JavaVersion.VERSION_17
         targetCompatibility = JavaVersion.VERSION_17
+        isCoreLibraryDesugaringEnabled = true
     }
     kotlinOptions {
         jvmTarget = "17"
@@ -51,6 +52,7 @@ android {
 }
 
 dependencies {
+    coreLibraryDesugaring("com.android.tools:desugar_jdk_libs:2.0.4")
     implementation("androidx.core:core-ktx:1.12.0")
     implementation("com.google.android.material:material:1.11.0")
     implementation("androidx.lifecycle:lifecycle-runtime-ktx:2.7.0")