fix: distinguish preparing from downloading in native worker progress

Prevent premature 'downloading' status before actual byte transfer
starts, and cache async provider values to avoid UI flicker during
queue library reloads.

Progress pipeline:
- StartItemProgress now initializes with 'preparing' status instead
  of 'downloading'
- SetItemProgress ignores synthetic pre-download progress updates
  while status is still 'preparing' (no byte data yet)
- DownloadService reads backend status field and propagates preparing/
  downloading/finalizing to native worker item snapshot
- Dart progress stream maps 'preparing' to DownloadStatus.downloading
  with progress 0.0 (indeterminate spinner)

Queue tab:
- Add _queueLibraryCountsCache and _queueLibraryPageDataCache to
  retain last successful data during FutureProvider refetches
- Prevents empty-state flash when loadedIndexVersion bumps trigger
  provider invalidation
- Caches trimmed to max 24 entries via FIFO eviction

android/app/src/main/kotlin/com/zarz/spotiflac/DownloadService.kt

@@ -564,12 +564,12 @@ class DownloadService : Service() {
                 nativeWorkerCurrentItemId = request.itemId
                 currentTrackName = request.trackName
                 currentArtistName = request.artistName
-                currentStatus = "downloading"
+                currentStatus = "preparing"
                 lastProgress = 0L
                 lastTotal = 0L
                 updateNotification(0, 0)
                 updateNativeWorkerItem(request.itemId) {
-                    it.status = "downloading"
+                    it.status = "preparing"
                     it.progress = 0.0
                     it.bytesReceived = 0L
                     it.bytesTotal = 0L
@@ -580,7 +580,7 @@ class DownloadService : Service() {
                     isRunning = true,
                     isPaused = false,
                     currentItemId = request.itemId,
-                    message = "Downloading",
+                    message = "Preparing",
                     settingsJson = settingsJson,
                     includeItems = true
                 )
@@ -1100,14 +1100,34 @@ class DownloadService : Service() {
             val root = JSONObject(raw)
             val items = root.optJSONObject("items") ?: return
             val progress = items.optJSONObject(itemId) ?: return
+            val backendStatus = progress.optString("status", "downloading")
             val bytesReceived = progress.optLong("bytes_received", 0L)
             val bytesTotal = progress.optLong("bytes_total", 0L)
+            if (backendStatus == "preparing") {
+                currentStatus = "preparing"
+                updateNativeWorkerItem(itemId) {
+                    it.status = "preparing"
+                    it.progress = 0.0
+                    it.bytesReceived = 0L
+                    it.bytesTotal = 0L
+                }
+                lastProgress = 0L
+                lastTotal = 0L
+                updateNotification(0L, 0L)
+                return
+            }
             val progressValue = if (bytesTotal > 0L) {
                 bytesReceived.toDouble() / bytesTotal.toDouble()
             } else {
                 progress.optDouble("progress", 0.0)
             }.coerceIn(0.0, 1.0)
+            currentStatus = if (backendStatus == "finalizing") {
+                "finalizing"
+            } else {
+                "downloading"
+            }
             updateNativeWorkerItem(itemId) {
+                it.status = currentStatus
                 it.progress = progressValue
                 it.bytesReceived = bytesReceived
                 it.bytesTotal = bytesTotal