gitignore: don't let a downloaded APK into history

Debug APKs get pulled into the working tree for emulator testing. They are ~57 MB
and come from CI artifacts, so they are never a source — but nothing stopped
`git add -A` from committing one permanently.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitignore

@@ -189,3 +189,10 @@ android/build/
 android/app/build/
 android/local.properties
 .kotlin/
+
+# Locally-downloaded APKs for emulator/device testing.
+#
+# CI builds these and attaches them to the run as artifacts; a copy sitting in
+# the working tree is a convenience, never a source. Ignored because they are
+# ~57 MB and `git add -A` would otherwise put one in history forever.
+*.apk