feat: add local-first private AI digest workflow

Migrate app code into canonical feature slices, add phone-only AI digest scheduling and review, wire local notification/background task support, and cover the flow with tests.

lib/core/config/app_config.dart

@@ -9,6 +9,11 @@ class AppConfig {
 
   static String? _baseUrlOverride;
 
+  static const String _defaultSentryDsn = String.fromEnvironment(
+    'SENTRY_DSN',
+    defaultValue: '',
+  );
+
   /// Returns the configured backend base URL.
   static String get backendBaseUrl {
     final String? override = _baseUrlOverride;
@@ -48,6 +53,9 @@ class AppConfig {
     defaultValue: 180,
   );
 
+  /// Optional Sentry DSN for release crash/error reporting.
+  static String get sentryDsn => _defaultSentryDsn.trim();
+
   /// Runtime override for backend URL (e.g. local settings screen).
   static void overrideBackendBaseUrl(String? baseUrl) {
     _baseUrlOverride = baseUrl;