fix: ntfy use JSON publish API (raw body was treated as attachment)

ntfy.sh now treats any request body (PUT/POST, text/plain) as a file
attachment, so messages never showed in the app. Switch to ntfy's JSON
publish API: POST / with {topic,title,message}.

version 1.7.1 -> 1.7.2 (code 14 -> 15)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/build.gradle.kts

@@ -13,8 +13,8 @@ android {
         applicationId = "com.a2i.forwarder"
         minSdk = 34
         targetSdk = 36
-        versionCode = 14
-        versionName = "1.7.1"
+        versionCode = 15
+        versionName = "1.7.2"
     }
 
     signingConfigs {