Warm up Coil ImageLoader off the main thread

The singleton ImageLoader factory reads cacheDir, so letting the first
AsyncImage build it lazily during composition tripped StrictMode's
DiskReadViolation on the UI thread. Pre-build it on the IO scope at startup.

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

@@ -356,6 +356,13 @@ class Amber :
         instance = this
         stats.createNotificationChannel()
         Thread.setDefaultUncaughtExceptionHandler(UnexpectedCrashSaver(crashReportCache, applicationIOScope))
+
+        // Build Coil's singleton ImageLoader off the main thread. Its factory
+        // (newImageLoader) touches cacheDir, so letting the first AsyncImage
+        // build it lazily during composition trips StrictMode's DiskReadViolation.
+        applicationIOScope.launch {
+            SingletonImageLoader.get(this@Amber)
+        }
     }
 
     private fun enableStrictMode() {