fix(events): show the venue-local concert time consistently

Backend sent the UTC `dateTime` (a 20:00 local show read as 08:00Z), and
Android's ZonedDateTime.parse choked on zone-less values → some rows had a
wrong time, others none at all.

- Backend: prefer localDate+localTime (venue-local, naive) over the UTC instant.
- Android: parse a naive LocalDateTime first, fall back to ZonedDateTime for
  older "…Z" values. Web already renders the naive string as-is.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

android/app/src/main/kotlin/com/novasonic/app/ui/component/ConcertsSheet.kt

@@ -21,6 +21,7 @@ import androidx.compose.ui.unit.dp
 import androidx.compose.ui.unit.sp
 import com.novasonic.app.api.model.ConcertEvent
 import com.novasonic.app.ui.theme.*
+import java.time.LocalDateTime
 import java.time.ZonedDateTime
 import java.time.format.DateTimeFormatter
 import java.util.Locale
@@ -165,12 +166,17 @@ private fun EventRow(event: ConcertEvent, onTicketClick: (String) -> Unit) {
     }
 }
 
+private val DATE_FMT = DateTimeFormatter.ofPattern("dd. MMM yyyy", Locale.GERMAN)
+private val TIME_FMT = DateTimeFormatter.ofPattern("HH:mm")
+
 private fun formatEventDateTime(datetime: String): Pair<String, String> {
     return try {
-        val zdt = ZonedDateTime.parse(datetime)
-        val date = zdt.format(DateTimeFormatter.ofPattern("dd. MMM yyyy", Locale.GERMAN))
-        val time = if (zdt.hour == 0 && zdt.minute == 0) ""
-                   else zdt.format(DateTimeFormatter.ofPattern("HH:mm"))
+        // Backend now sends a naive venue-local datetime ("2026-09-16T20:00:00"); older/zoned
+        // values ("…Z") still parse via the ZonedDateTime fallback.
+        val ldt = runCatching { LocalDateTime.parse(datetime) }
+            .getOrElse { ZonedDateTime.parse(datetime).toLocalDateTime() }
+        val date = ldt.format(DATE_FMT)
+        val time = if (ldt.hour == 0 && ldt.minute == 0) "" else ldt.format(TIME_FMT)
         date to time
     } catch (_: Exception) {
         datetime.take(10) to ""