Warm up tzdata and cache DateTimeFormatters off the main thread

The first ZoneId.systemDefault() call lazily mmaps the tzdata file
(ZoneInfoDb class init), which tripped StrictMode DiskReadViolation
(~169 ms) on the main thread during LazyColumn composition in
ApplicationsScreen. Mirror the Coil ImageLoader pre-warm: force the
load on applicationIOScope in Amber.onCreate, and cache the two
DateTimeFormatter instances instead of re-parsing the pattern per item.

app/src/main/java/com/greenart7c3/nostrsigner/Amber.kt

@@ -40,6 +40,7 @@ import com.greenart7c3.nostrsigner.database.LogDatabase
 import com.greenart7c3.nostrsigner.models.Account
 import com.greenart7c3.nostrsigner.models.AmberSettings
 import com.greenart7c3.nostrsigner.models.FeedbackType
+import com.greenart7c3.nostrsigner.models.TimeUtils as AmberTimeUtils
 import com.greenart7c3.nostrsigner.models.TorMode
 import com.greenart7c3.nostrsigner.models.UpdateCheckFrequency
 import com.greenart7c3.nostrsigner.okhttp.HttpClientManager
@@ -354,6 +355,13 @@ class Amber :
         applicationIOScope.launch {
             SingletonImageLoader.get(this@Amber)
         }
+
+        // The first ZoneId.systemDefault() call lazily mmaps tzdata from disk.
+        // Force it here so it never happens on the main thread during the first
+        // timestamp formatted in composition (StrictMode DiskReadViolation).
+        applicationIOScope.launch {
+            AmberTimeUtils.warmUp()
+        }
     }
 
     private fun enableStrictMode() {

app/src/main/java/com/greenart7c3/nostrsigner/models/TimeUtils.kt

@@ -6,15 +6,29 @@ import java.time.ZoneId
 import java.time.format.DateTimeFormatter
 
 object TimeUtils {
+    // DateTimeFormatter is immutable and thread-safe; building it parses the
+    // pattern on every call, so cache the two instances used during composition.
+    private val formatter = DateTimeFormatter.ofPattern("HH:mm - dd MMM")
+    private val formatterWithSeconds = DateTimeFormatter.ofPattern("HH:mm:ss - dd MMM")
+
+    /**
+     * The first ZoneId.systemDefault() call in the process lazily loads the
+     * tzdata file from disk (ZoneInfoDb's class initializer). Calling this from
+     * a background thread at app start keeps that disk read off the main
+     * thread, where it would otherwise trip StrictMode and jank the frame that
+     * first formats a timestamp.
+     */
+    fun warmUp() {
+        LocalDateTime.ofInstant(Instant.ofEpochMilli(System.currentTimeMillis()), ZoneId.systemDefault()).format(formatter)
+    }
+
     fun formatLongToCustomDateTime(longValue: Long): String {
         val dateTime = LocalDateTime.ofInstant(Instant.ofEpochMilli(longValue), ZoneId.systemDefault())
-        val formatter = DateTimeFormatter.ofPattern("HH:mm - dd MMM")
         return dateTime.format(formatter)
     }
 
     fun formatLongToCustomDateTimeWithSeconds(longValue: Long): String {
         val dateTime = LocalDateTime.ofInstant(Instant.ofEpochMilli(longValue), ZoneId.systemDefault())
-        val formatter = DateTimeFormatter.ofPattern("HH:mm:ss - dd MMM")
-        return dateTime.format(formatter)
+        return dateTime.format(formatterWithSeconds)
     }
 }