Wear UI: intercept system back so Messages/Conversation/Accounts pop within app

Add BackHandler in WearApp: back from thread: -> messages, messages/accounts
-> dialer. At the root dialer route the handler is disabled so back finishes
the Activity normally. Fixes back-from-Messages dropping to the clock face.

wear/src/main/java/com/tutpro/baresip/wear/MainActivity.kt

@@ -8,6 +8,7 @@ import android.os.Build
 import android.os.Bundle
 import android.view.KeyEvent
 import androidx.activity.ComponentActivity
+import androidx.activity.compose.BackHandler
 import androidx.activity.compose.setContent
 import androidx.activity.result.contract.ActivityResultContracts
 import androidx.compose.foundation.layout.*
@@ -189,6 +190,17 @@ class MainActivity : ComponentActivity() {
 fun WearApp(activity: MainActivity) {
     // Show the in-call screen whenever there is an active or incoming call.
     val hasCall by remember { derivedStateOf { CallState.calls.isNotEmpty() } }
+    // Intercept the system back gesture so navigation stays inside the app
+    // (Messages/Conversation/Accounts screens pop to their parent instead of
+    // finishing the Activity and dropping to the clock face). At the root
+    // "dialer" route the handler is disabled, so back behaves normally.
+    BackHandler(enabled = activity.route.value != "dialer") {
+        activity.route.value = when {
+            activity.route.value.startsWith("thread:") -> "messages"
+            activity.route.value == "messages" || activity.route.value == "accounts" -> "dialer"
+            else -> "dialer"
+        }
+    }
     MaterialTheme {
         Scaffold(timeText = { TimeText() }) {
             when {