Fix MainActivity appearing behind bunker notification bottom sheet

SignerActivity is a transparent modal bottom sheet. Without its own
taskAffinity it was launched inside Amber's existing task (containing
MainActivity), making MainActivity visible through the transparent scrim.

Add android:taskAffinity="" to SignerActivity in both manifests so it
always runs in its own isolated task. Also add FLAG_ACTIVITY_NEW_TASK
to the notification PendingIntent to enforce the new-task launch.

https://claude.ai/code/session_01XnocJeiPCgdXRq7syUxwpG

app/src/free/AndroidManifest.xml

@@ -39,6 +39,7 @@
         <activity
             android:name=".SignerActivity"
             android:exported="true"
+            android:taskAffinity=""
             android:theme="@style/Theme.NostrSigner.Modal">
             <intent-filter>
                 <action android:name="com.greenart7c3.nostrsigner.MAIN_ACTIVITY" />

app/src/main/AndroidManifest.xml

@@ -71,6 +71,7 @@
         <activity
             android:name=".SignerActivity"
             android:exported="true"
+            android:taskAffinity=""
             android:theme="@style/Theme.NostrSigner.Modal">
             <intent-filter>
                 <action android:name="com.greenart7c3.nostrsigner.MAIN_ACTIVITY" />

app/src/main/java/com/greenart7c3/nostrsigner/service/NotificationUtils.kt

@@ -151,7 +151,7 @@ object NotificationUtils {
 
         val contentIntent = Intent(applicationContext, SignerActivity::class.java)
         contentIntent.putExtra("route", Route.IncomingRequest.route)
-        contentIntent.addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP or Intent.FLAG_ACTIVITY_CLEAR_TOP)
+        contentIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK or Intent.FLAG_ACTIVITY_SINGLE_TOP or Intent.FLAG_ACTIVITY_CLEAR_TOP)
         val contentPendingIntent =
             PendingIntent.getActivity(
                 applicationContext,