fix(android): read versionCode/versionName from flutter pubspec instead of hardcoded values

Was hardcoded to versionCode=17, versionName="1.6.1(p)" — overriding
pubspec.yaml and causing Play Store rejection. Now reads flutter.versionCode
and flutter.versionName so pubspec.yaml is the single source of truth.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

android/app/build.gradle.kts

@@ -22,8 +22,8 @@ android {
         applicationId = "com.sicherhaven.eventify"
         minSdk = flutter.minSdkVersion
         targetSdk = flutter.targetSdkVersion
-        versionCode = 17
-        versionName = "1.6.1(p)"
+        versionCode = flutter.versionCode
+        versionName = flutter.versionName
     }
 
     // ---------- SIGNING CONFIG ----------