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/build.gradle

@@ -0,0 +1,29 @@
+// Top-level build file where you can add configuration options common to all sub-projects/modules.
+
+buildscript {
+    
+    repositories {
+        google()
+        mavenCentral()
+    }
+    dependencies {
+        classpath 'com.android.tools.build:gradle:8.7.2'
+        classpath 'com.google.gms:google-services:4.4.2'
+
+        // NOTE: Do not place your application dependencies here; they belong
+        // in the individual module build.gradle files
+    }
+}
+
+apply from: "variables.gradle"
+
+allprojects {
+    repositories {
+        google()
+        mavenCentral()
+    }
+}
+
+task clean(type: Delete) {
+    delete rootProject.buildDir
+}