feat(artists): show provider-neutral upcoming concerts

lib/models/artist_concert.dart

@@ -0,0 +1,83 @@
+import 'package:spotiflac_android/utils/string_utils.dart';
+import 'package:timezone/data/latest.dart' as tz_data;
+import 'package:timezone/timezone.dart' as tz;
+
+/// Provider-neutral event metadata. Calendar labels always use the venue's
+/// local time, never the listener's device time zone.
+class ArtistConcert {
+  const ArtistConcert({
+    required this.id,
+    required this.location,
+    required this.venue,
+    required this.date,
+    required this.hasTime,
+    this.url,
+  });
+
+  final String id;
+  final String location;
+  final String venue;
+  final DateTime date;
+  final bool hasTime;
+  final String? url;
+
+  static bool _timeZonesReady = false;
+
+  static List<ArtistConcert> parseList(Object? value) {
+    if (value is! List) return const [];
+    final concerts = <String, ArtistConcert>{};
+    for (final item in value.take(500)) {
+      if (item is! Map<String, dynamic>) continue;
+      String text(String key) =>
+          item[key] is String ? (item[key] as String).trim() : '';
+      final rawDate = text('start_at');
+      final match = RegExp(
+        r'^(\d{4})-(\d{2})-(\d{2})(?:T(\d{2}):(\d{2})(?::\d{2}(?:\.\d+)?)?(Z|[+-]\d{2}:?\d{2})?)?$',
+      ).firstMatch(rawDate);
+      final instant = DateTime.tryParse(rawDate);
+      final location = text('location');
+      if (match == null || instant == null || location.isEmpty) continue;
+      final year = int.parse(match[1]!);
+      final month = int.parse(match[2]!);
+      final day = int.parse(match[3]!);
+      final hour = int.parse(match[4] ?? '0');
+      final minute = int.parse(match[5] ?? '0');
+      var date = DateTime.utc(year, month, day, hour, minute);
+      // DateTime.parse normalizes impossible dates; do not show invented days.
+      if (date.year != year ||
+          date.month != month ||
+          date.day != day ||
+          hour > 23 ||
+          minute > 59) {
+        continue;
+      }
+      final zone = text('time_zone');
+      if (zone.isNotEmpty && match[6] != null) {
+        if (!_timeZonesReady) {
+          tz_data.initializeTimeZones();
+          _timeZonesReady = true;
+        }
+        try {
+          date = tz.TZDateTime.from(instant, tz.getLocation(zone));
+        } on tz.LocationNotFoundException {
+          // Preserve the supplied wall-clock date if the zone is unknown.
+        }
+      }
+      final id = text('id');
+      final venue = text('venue');
+      final key = id.isEmpty ? '$rawDate|$location|$venue' : id;
+      concerts.putIfAbsent(
+        key,
+        () => ArtistConcert(
+          id: key,
+          location: location,
+          venue: venue,
+          date: date,
+          hasTime: match[4] != null,
+          url: normalizeRemoteHttpUrl(text('url')),
+        ),
+      );
+    }
+    return concerts.values.toList()..sort((a, b) => a.date.compareTo(b.date));
+  }
+}