Add module build configs and platform expect/actual stubs

- composeApp/build.gradle.kts: KMP targets (android + iOS), all dependencies
- androidApp/build.gradle.kts: Android app with compose compiler
- Platform.kt expect/actual for Android and iOS
- Android resources: launcher icons, themes, manifest

Co-Authored-By: Claude Opus 4.6 (1M context)

Signed-off-by: Avinal Kumar <avinal.xlvii@gmail.com>

androidApp/proguard-rules.pro

@@ -0,0 +1,17 @@
+# Ktor
+-keep class io.ktor.** { *; }
+-dontwarn io.ktor.**
+
+# kotlinx.serialization
+-keepattributes *Annotation*, InnerClasses
+-dontnote kotlinx.serialization.AnnotationsKt
+-keepclassmembers class kotlinx.serialization.json.** { *** Companion; }
+-keepclasseswithmembers class kotlinx.serialization.json.** { kotlinx.serialization.KSerializer serializer(...); }
+-keep,includedescriptorclasses class com.avinal.memos.**$$serializer { *; }
+-keepclassmembers class com.avinal.memos.** { *** Companion; }
+-keepclasseswithmembers class com.avinal.memos.** { kotlinx.serialization.KSerializer serializer(...); }
+
+# Room
+-keep class * extends androidx.room.RoomDatabase
+-keep @androidx.room.Entity class *
+-dontwarn androidx.room.paging.**