Add release signing config and fix missing INTERNET permission

- Configure release signing via key.properties + Gradle Kotlin DSL
- Add INTERNET and ACCESS_NETWORK_STATE permissions to AndroidManifest
  (Flutter injects INTERNET automatically in debug builds but not release,
  causing DNS failures in sideloaded APKs)
- Restore android/.gitignore entries lost during signing setup
- Gitignore key.properties and fabled-release-key.jks

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

android/.gitignore

@@ -7,8 +7,6 @@ gradle-wrapper.jar
 GeneratedPluginRegistrant.java
 .cxx/
 
-# Remember to never publicly share your keystore.
-# See https://flutter.dev/to/reference-keystore
+# Signing secrets — never commit these
 key.properties
-**/*.keystore
-**/*.jks
+fabled-release-key.jks