UApp: fix Koin readiness flag, first-run race and context accessor

- initiated is now owned by startKoin() alone; setting it before super.onCreate()
  made the NavigationActivity Koin-restart fallback unreachable
- isFirstRun is computed on the main thread: NavigationActivity.onCreate can run
  before the background initialization completed and read a stale 'false',
  silently skipping the welcome dialog on first install
- applicationContext() fails with a descriptive error instead of a bare NPE
  when accessed before Application.onCreate

ultrasonic/src/main/kotlin/org/moire/ultrasonic/app/UApp.kt

@@ -61,7 +61,6 @@ class UApp :
     var setupDialogDisplayed = false
 
     override fun onCreate() {
-        initiated = true
         super.onCreate()
 
         if (BuildConfig.DEBUG) {
@@ -70,6 +69,11 @@ class UApp :
 
         Timber.d("onCreate called")
 
+        // This flag flips once per install and NavigationActivity reads it while creating
+        // its UI, which can happen before the background initialization below completes.
+        // Reading it only on the main thread avoids serving a stale "false" on first run.
+        isFirstRun = Util.isFirstRun()
+
         // In general we should not access the settings from the main thread to avoid blocking...
         ioScope.launch {
             if (Settings.debugLogToFile) {
@@ -79,7 +83,6 @@ class UApp :
             // Populate externalFilesDir early
             FileUtil.cachedUltrasonicDirectory = FileUtil.ultrasonicDirectory
             Storage.mediaRoot.value
-            isFirstRun = Util.isFirstRun()
         }
 
         startKoin()
@@ -113,6 +116,8 @@ class UApp :
     }
 
     internal fun startKoin() {
+        // "Koin is ready" is owned by startKoin alone; NavigationActivity relies on this
+        // flag to re-start Koin if the Application was recreated without a completed onCreate.
         initiated = true
         startKoin {
             // Sometimes Koin breaks when Kotlin version is upgraded,
@@ -142,7 +147,10 @@ class UApp :
     companion object {
         var instance: UApp? = null
 
-        fun applicationContext(): Context = instance!!.applicationContext
+        fun applicationContext(): Context =
+            requireNotNull(instance) {
+                "UApp.instance accessed before Application.onCreate"
+            }.applicationContext
     }
 }