Scaffold native Android build and enforce Rust boundaries (#89)

android/core/src/main/AndroidManifest.xml

@@ -0,0 +1 @@
+<manifest />

android/core/src/main/kotlin/de/rfc1437/ironstorage/core/ShellRepository.kt

@@ -0,0 +1,23 @@
+package de.rfc1437.ironstorage.core
+
+import kotlinx.coroutines.flow.StateFlow
+
+enum class ShellState { Loading, Empty, Ready, Locked, Error }
+enum class ShellTab { Home, Passwords, Search, Totp, Preferences }
+
+data class ShellPage(
+    val tab: ShellTab,
+    val title: String,
+    val stateTitle: String,
+    val detail: String
+)
+data class ShellSnapshot(
+    val state: ShellState,
+    val pages: List<ShellPage>,
+    val selectedTab: ShellTab = ShellTab.Home
+)
+
+interface ShellRepository {
+    val snapshot: StateFlow<ShellSnapshot>
+    fun selectTab(tab: ShellTab)
+}