feat(android): LibraryViewModel + UiState + MainDispatcherExtension (M8 phase 5.3)

First Hilt-injected ViewModel + sealed UiState pattern.

LibraryUiState (sealed interface): Loading / Empty / Success / Error.
The cases are exhaustive so Compose `when` blocks the compiler checks.

LibraryViewModel:
  - combine(observeArtists, observeAlbums) → Success/Empty decision
  - .catch translates upstream Flow exceptions to UiState.Error
  - .stateIn(viewModelScope, WhileSubscribed(5_000), Loading) — the
    standard Compose-friendly pattern; subscriptions tear down 5s after
    the last collector to ride out config changes without hanging the
    DAO Flow forever.

MainDispatcherExtension — JUnit 5 equivalent of the JUnit 4
MainDispatcherRule pattern (audit-deferred item; trigger met). Swaps
Dispatchers.Main for UnconfinedTestDispatcher in beforeEach +
resetMain in afterEach. Apply with `@ExtendWith`.

LibraryViewModelTest covers all four UiState cases — initial Loading,
empty cache (Empty), populated cache (Success), and an upstream Flow
exception (Error). MockK for the repo, Turbine for the Flow assertions.

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

android/app/src/main/java/com/fabledsword/minstrel/library/ui/LibraryUiState.kt

@@ -0,0 +1,26 @@
+package com.fabledsword.minstrel.library.ui
+
+import com.fabledsword.minstrel.models.AlbumRef
+import com.fabledsword.minstrel.models.ArtistRef
+
+/**
+ * UI state for the Library screen. Sealed interface — each composable
+ * branch handles exactly the cases it cares about; the compiler
+ * enforces exhaustiveness.
+ *
+ * - Loading: initial state before the first DAO emission.
+ * - Empty: DAO emitted but the cache holds no artists and no albums
+ *   (fresh install, never synced).
+ * - Success: DAO emitted with content.
+ * - Error: an exception bubbled from the DAO/repository layer; the
+ *   message is surfaced as-is in the empty-state copy.
+ */
+sealed interface LibraryUiState {
+    data object Loading : LibraryUiState
+    data object Empty : LibraryUiState
+    data class Success(
+        val artists: List<ArtistRef>,
+        val albums: List<AlbumRef>,
+    ) : LibraryUiState
+    data class Error(val message: String) : LibraryUiState
+}

android/app/src/main/java/com/fabledsword/minstrel/library/ui/LibraryViewModel.kt

@@ -0,0 +1,40 @@
+package com.fabledsword.minstrel.library.ui
+
+import androidx.lifecycle.ViewModel
+import androidx.lifecycle.viewModelScope
+import com.fabledsword.minstrel.library.data.LibraryRepository
+import dagger.hilt.android.lifecycle.HiltViewModel
+import kotlinx.coroutines.flow.SharingStarted
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.catch
+import kotlinx.coroutines.flow.combine
+import kotlinx.coroutines.flow.stateIn
+import javax.inject.Inject
+
+private const val SHARE_STOP_TIMEOUT_MS = 5_000L
+
+@HiltViewModel
+class LibraryViewModel @Inject constructor(
+    private val repository: LibraryRepository,
+) : ViewModel() {
+
+    val uiState: StateFlow<LibraryUiState> =
+        combine(
+            repository.observeArtists(),
+            repository.observeAlbums(),
+        ) { artists, albums ->
+            if (artists.isEmpty() && albums.isEmpty()) {
+                LibraryUiState.Empty
+            } else {
+                LibraryUiState.Success(artists, albums)
+            }
+        }
+            .catch { e ->
+                emit(LibraryUiState.Error(e.message ?: "Library load failed"))
+            }
+            .stateIn(
+                scope = viewModelScope,
+                started = SharingStarted.WhileSubscribed(SHARE_STOP_TIMEOUT_MS),
+                initialValue = LibraryUiState.Loading,
+            )
+}