fix(mobile): use correct timezone displayed in the info sheet (#24310)

* fixed the timezone issue in the Immich mobile app's metadata sheet to match the web app's behavior

* format dart

* now uses the shared applyTimezoneOffset() utility function from mobile/lib/utils/timezone.dart

* add tests

---------

Co-authored-by: Alex <alex.tran1502@gmail.com>

mobile/lib/utils/timezone.dart

@@ -0,0 +1,35 @@
+import 'package:timezone/timezone.dart';
+
+/// Applies timezone conversion to a DateTime using EXIF timezone information.
+///
+/// This function handles two timezone formats:
+/// 1. Named timezone locations (e.g., "Asia/Hong_Kong")
+/// 2. UTC offset format (e.g., "UTC+08:00", "UTC-05:00")
+///
+/// Returns a tuple of (adjusted DateTime, timezone offset Duration)
+(DateTime, Duration) applyTimezoneOffset({required DateTime dateTime, required String? timeZone}) {
+  DateTime dt = dateTime.toUtc();
+
+  if (timeZone == null) {
+    return (dt, dt.timeZoneOffset);
+  }
+
+  try {
+    // Try to get timezone location from database
+    final location = getLocation(timeZone);
+    dt = TZDateTime.from(dt, location);
+    return (dt, dt.timeZoneOffset);
+  } on LocationNotFoundException {
+    // Handle UTC offset format (e.g., "UTC+08:00")
+    RegExp re = RegExp(r'^utc(?:([+-]\d{1,2})(?::(\d{2}))?)?$', caseSensitive: false);
+    final m = re.firstMatch(timeZone);
+    if (m != null) {
+      final duration = Duration(hours: int.parse(m.group(1) ?? '0'), minutes: int.parse(m.group(2) ?? '0'));
+      dt = dt.add(duration);
+      return (dt, duration);
+    }
+  }
+
+  // If timezone is invalid, return UTC
+  return (dt, dt.timeZoneOffset);
+}