feat(android): scaffold Capacitor 8 native project + hardware Back button

Add @capacitor/{core,android,app,cli,assets} to ui/, a capacitor.config.ts (appId ru.eruditgame.app, appName Эрудит, webDir dist — bundle model, no server.url), and the generated ui/android/ Gradle project (tracked; build outputs and the machine-specific local.properties gitignored, keystore patterns un-commented so signing material can never be committed). Wire the Android hardware Back button in ui/src/lib/native.ts behind a dynamic @capacitor/app import (web/mock bundles never load it), called from App.svelte onMount and reusing routeDepth for the navigation-root check. Whitelist sharp in pnpm-workspace.yaml for @capacitor/assets icon generation.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

ui/android/variables.gradle

@@ -0,0 +1,16 @@
+ext {
+    minSdkVersion = 24
+    compileSdkVersion = 36
+    targetSdkVersion = 36
+    androidxActivityVersion = '1.11.0'
+    androidxAppCompatVersion = '1.7.1'
+    androidxCoordinatorLayoutVersion = '1.3.0'
+    androidxCoreVersion = '1.17.0'
+    androidxFragmentVersion = '1.8.9'
+    coreSplashScreenVersion = '1.2.0'
+    androidxWebkitVersion = '1.14.0'
+    junitVersion = '4.13.2'
+    androidxJunitVersion = '1.3.0'
+    androidxEspressoCoreVersion = '3.7.0'
+    cordovaAndroidVersion = '14.0.1'
+}