feat(v1.5.0): Complete i18n implementation + Language Selector feature

- Added comprehensive English (strings.xml) and German (strings-de.xml) localization with 400+ strings
- Created new LanguageSettingsScreen with System Default, English, and German options
- Fixed hardcoded German notification toasts in MainActivity and ComposeMainActivity
- Integrated Language selector in Settings as top-level menu item
- Changed ComposeSettingsActivity from ComponentActivity to AppCompatActivity for AppCompatDelegate compatibility
- Added locales_config.xml for Android 13+ Per-App Language support
- Updated Extensions.kt with i18n-aware timestamp formatting (toReadableTime with context)
- Translated all UI strings including settings, toasts, notifications, and error messages
- Added dynamic language display in SettingsMainScreen showing current language

Fixes:
- Notification permission toast now respects system language setting
- Activity correctly restarts when language is changed
- All string formatting with parameters properly localized

Migration:
- MainViewModel: All toast messages now use getString()
- SettingsViewModel: All toast and dialog messages localized
- NotificationHelper: Notification titles and messages translated
- UrlValidator: Error messages now accept Context parameter for translation
- NoteCard, DeleteConfirmationDialog, SyncStatusBanner: All strings externalized

Testing completed on device with both EN and DE locale switching.
Closes #5

android/app/src/main/AndroidManifest.xml

@@ -22,6 +22,7 @@
         android:fullBackupContent="@xml/backup_rules"
         android:icon="@mipmap/ic_launcher"
         android:label="@string/app_name"
+        android:localeConfig="@xml/locales_config"
         android:roundIcon="@mipmap/ic_launcher_round"
         android:supportsRtl="true"
         android:theme="@style/Theme.SimpleNotes"