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/android/variables.gradle

@@ -0,0 +1,16 @@
+ext {
+    minSdkVersion = 23
+    compileSdkVersion = 35
+    targetSdkVersion = 35
+    androidxActivityVersion = '1.9.2'
+    androidxAppCompatVersion = '1.7.0'
+    androidxCoordinatorLayoutVersion = '1.2.0'
+    androidxCoreVersion = '1.15.0'
+    androidxFragmentVersion = '1.8.4'
+    coreSplashScreenVersion = '1.0.1'
+    androidxWebkitVersion = '1.12.1'
+    junitVersion = '4.13.2'
+    androidxJunitVersion = '1.2.1'
+    androidxEspressoCoreVersion = '3.6.1'
+    cordovaAndroidVersion = '10.1.1'
+}