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/app/capacitor.build.gradle

@@ -0,0 +1,22 @@
+// DO NOT EDIT THIS FILE! IT IS GENERATED EACH TIME "capacitor update" IS RUN
+
+android {
+  compileOptions {
+      sourceCompatibility JavaVersion.VERSION_21
+      targetCompatibility JavaVersion.VERSION_21
+  }
+}
+
+apply from: "../capacitor-cordova-android-plugins/cordova.variables.gradle"
+dependencies {
+    implementation project(':capacitor-app')
+    implementation project(':capacitor-keyboard')
+    implementation project(':capacitor-preferences')
+    implementation project(':capacitor-status-bar')
+
+}
+
+
+if (hasProperty('postBuildExtras')) {
+  postBuildExtras()
+}