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/app/proguard-rules.pro

@@ -0,0 +1,12 @@
+# Keep kotlinx.serialization metadata
+-keepattributes *Annotation*, InnerClasses
+-dontnote kotlinx.serialization.**
+-keepclassmembers class app.voltplan.cable.** {
+    *** Companion;
+}
+-keepclasseswithmembers class app.voltplan.cable.** {
+    kotlinx.serialization.KSerializer serializer(...);
+}
+# Retrofit
+-keep,allowobfuscation,allowshrinking interface retrofit2.Call
+-keep,allowobfuscation,allowshrinking class retrofit2.Response