chore(android): untrack local Gradle/build cache, extend .gitignore

Previous commit (0b72827) accidentally swept the local
android/.gradle, android/build, android/app/build, and
android/local.properties into the index via `git add android/`.
Root .gitignore only had entries for flutter_client/android/
paths, not the new native android/ tree.

Removes the cached files via `git rm --cached` and extends
.gitignore to cover the native Android Studio output dirs
(.gradle, .kotlin, .idea, build, app/build, local.properties,
*.iml). Source code is unaffected — only build-output and IDE
artifacts get untracked.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -57,3 +57,13 @@ flutter_client/android/app/build/
 flutter_client/android/local.properties
 flutter_client/android/key.properties
 flutter_client/*.iml
+
+# Native Android (Kotlin/Compose) — M8 rewrite
+android/.gradle/
+android/.kotlin/
+android/.idea/
+android/build/
+android/app/build/
+android/local.properties
+android/*.iml
+android/app/*.iml