Ship Android app by wrapping CRA build with Capacitor (#20)

Add Capacitor 7 scaffolding (config, android/, npm scripts, .env.mobile),
native back-button/safe-area chrome, icons/splash, and signing docs so
llm-fe ships one codebase to web and Android.

llm-fe/.gitignore

@@ -11,12 +11,25 @@
 # production
 /build
 
+# Capacitor / Android Gradle artifacts (keep android/ source; ignore local builds)
+/android/.gradle/
+/android/app/build/
+/android/build/
+/android/captures/
+/android/local.properties
+/android/*.iml
+/android/app/*.iml
+/android/.idea/
+*.keystore
+*.jks
+
 # misc
 .DS_Store
 .env.local
 .env.development.local
 .env.test.local
 .env.production.local
+.env.mobile.local
 
 npm-debug.log*
 yarn-debug.log*