Add native Kotlin/Compose Android port

Full feature-parity port of the iOS app under ./android (package
app.voltplan.cable): Systems, bottom-nav system detail (Overview,
Components, Batteries, Chargers), calculator/loads, battery & charger
editors, overview with runtime/charge goals, Bill of Materials with PDF
export, VoltPlan PocketBase component library, and Aptabase analytics.
Room persistence, 5-language localization. Verified to build (assembleDebug).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

android/.gitignore

@@ -0,0 +1,13 @@
+*.iml
+.gradle/
+/local.properties
+/.idea/
+.DS_Store
+/build
+/captures
+.externalNativeBuild
+.cxx
+/app/build/
+
+# Local build artifacts
+*.apk