build(android): configure release signing with keystore

Replaces default debug signing with a dedicated release keystore loaded via android/key.properties. Fixes the cross-host signature mismatch that broke adb install -r. keystore and key.properties are gitignored (signing identity must not be committed).

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

.gitignore

@@ -46,6 +46,10 @@ app.*.map.json
 /android/app/release
 /android/build/
 
+# Release signing keystore (do NOT commit — contains signing identity)
+/android/key.properties
+/android/app/key.jks
+
 # AI Agents
 .claude/
 .agents/