refactor: use injected ioDispatcher and ApplicationCoroutineScope

Replaces two coroutine anti-patterns flagged by the architecture audit:

* FirmwareUpdateViewModel.onCleared() was using GlobalScope.launch to
  run cleanup work that must outlive the ViewModel scope. Introduce a
  new ApplicationCoroutineScope marker interface + Koin @Single binding
  in core/common, backed by SupervisorJob() + ioDispatcher, and inject
  it into the VM. NonCancellable is preserved so shutdown work still
  cannot be interrupted. Verified end-to-end by the KoinVerificationTest
  full-graph check.
* Six sites in feature/settings (android+jvm LogExporter/PrefExporter)
  and core/ui (android+jvm PlatformUtils) were hardcoding Dispatchers.IO.
  Switch to the project's injectable ioDispatcher property so tests
  can substitute a test dispatcher without reflection.

Dispatchers.Main sites in LogExporter are intentionally left alone —
they are fully test-swappable via Dispatchers.setMain() and there is
no existing mainDispatcher property to inject.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

core/common/src/commonMain/kotlin/org/meshtastic/core/common/di/ApplicationCoroutineScope.kt

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2026 Meshtastic LLC
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.meshtastic.core.common.di
+
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.SupervisorJob
+import org.koin.core.annotation.Single
+import org.meshtastic.core.common.util.ioDispatcher
+
+/**
+ * A process-wide [CoroutineScope] that outlives individual ViewModels and UI components.
+ *
+ * Use this scope for fire-and-forget cleanup work that must continue after a ViewModel's own scope has been cancelled
+ * (for example, deleting temporary files in `onCleared()`). Backed by a [SupervisorJob] so failures in one child do not
+ * cancel siblings, and by [ioDispatcher] so work runs off the main thread.
+ *
+ * Prefer scoping work to a more specific scope (like `viewModelScope`) whenever possible; this scope is an escape hatch
+ * and should be used sparingly.
+ */
+interface ApplicationCoroutineScope : CoroutineScope
+
+@Single(binds = [ApplicationCoroutineScope::class])
+internal class ApplicationCoroutineScopeImpl : ApplicationCoroutineScope {
+    override val coroutineContext = SupervisorJob() + ioDispatcher
+}

core/ui/src/androidMain/kotlin/org/meshtastic/core/ui/util/PlatformUtils.kt

@@ -37,12 +37,12 @@ import androidx.lifecycle.compose.LifecycleEventEffect
 import co.touchlab.kermit.Logger
 import com.eygraber.uri.toAndroidUri
 import com.eygraber.uri.toKmpUri
-import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.withContext
 import org.jetbrains.compose.resources.StringResource
 import org.jetbrains.compose.resources.getString
 import org.meshtastic.core.common.gpsDisabled
 import org.meshtastic.core.common.util.CommonUri
+import org.meshtastic.core.common.util.ioDispatcher
 import java.net.URLEncoder
 
 @Composable
@@ -146,7 +146,7 @@ actual fun rememberReadTextFromUri(): suspend (uri: CommonUri, maxChars: Int) ->
     val context = LocalContext.current
     return remember(context) {
         { uri, maxChars ->
-            withContext(Dispatchers.IO) {
+            withContext(ioDispatcher) {
                 @Suppress("TooGenericExceptionCaught")
                 try {
                     val androidUri = uri.toAndroidUri()

core/ui/src/jvmMain/kotlin/org/meshtastic/core/ui/util/PlatformUtils.kt

@@ -20,10 +20,10 @@ package org.meshtastic.core.ui.util
 
 import androidx.compose.runtime.Composable
 import co.touchlab.kermit.Logger
-import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.withContext
 import org.jetbrains.compose.resources.StringResource
 import org.meshtastic.core.common.util.CommonUri
+import org.meshtastic.core.common.util.ioDispatcher
 import java.awt.Desktop
 import java.awt.FileDialog
 import java.awt.Frame
@@ -89,7 +89,7 @@ actual fun rememberOpenFileLauncher(onUriReceived: (CommonUri?) -> Unit): (mimeT
 /** JVM — Reads text from a file URI. */
 @Composable
 actual fun rememberReadTextFromUri(): suspend (uri: CommonUri, maxChars: Int) -> String? = { uri, maxChars ->
-    withContext(Dispatchers.IO) {
+    withContext(ioDispatcher) {
         @Suppress("TooGenericExceptionCaught")
         try {
             val file = File(URI(uri.toString()))