refactor(android): add generic shared/UiState<T> sealed type

android/app/src/main/java/com/fabledsword/minstrel/shared/UiState.kt

@@ -0,0 +1,22 @@
+package com.fabledsword.minstrel.shared
+
+/**
+ * Shared cache-first UI state for a screen / tab that resolves to a
+ * single payload. Mirrors the shape every screen had reinvented:
+ *  - [Loading]  cold start, before either the cache or the refresh
+ *               has emitted anything
+ *  - [Empty]    the cache + refresh both succeeded with no data
+ *  - [Success]  the cache (or refresh) has a payload — render it
+ *  - [Error]    no cached data + the refresh failed; UI surfaces
+ *               [message] from `ErrorCopy.fromThrowable`
+ *
+ * Each screen / tab parameterizes [T] with whatever it actually
+ * renders (`HomeSections`, `List<PlaylistRef>`, etc.), so a single
+ * generic replaces the per-screen sealed interface declarations.
+ */
+sealed interface UiState<out T> {
+    data object Loading : UiState<Nothing>
+    data object Empty : UiState<Nothing>
+    data class Success<T>(val data: T) : UiState<T>
+    data class Error(val message: String) : UiState<Nothing>
+}