One MainActivity, ever: a stacked instance made hang-up a no-op

The default launch mode stacked a second MainActivity — with a second
ViewModel, session and CallManager — whenever the app was re-entered.
The visible screen's Hang up then commanded a manager with no call while
the real call rang on underneath, and the peer never got the END.
singleTask, plus a 30s connect guard so an answered call whose ICE never
finds a path gives up out loud on both screens instead of sitting in
Connecting forever.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/main/AndroidManifest.xml

@@ -55,9 +55,20 @@
           presents as a corrupt install rather than as a missing key.
         -->
 
+        <!--
+          🔴 singleTask is load-bearing, not cosmetic. The default launch mode
+          stacks a NEW MainActivity — with a NEW ViewModel, session, poll loop
+          and CallManager — every time the app is entered through a
+          notification or a fresh launcher intent. Two instances means the
+          visible screen's Hang up commands a CallManager that has no call,
+          while the real call keeps ringing in the instance underneath: the
+          peer never receives the END, and "I ended the call but their phone
+          kept going" is exactly how it presents.
+        -->
         <activity
             android:name=".app.MainActivity"
             android:exported="true"
+            android:launchMode="singleTask"
             android:windowSoftInputMode="adjustResize">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />