feat(downloads): browsable downloaded library with on-disk usage

Replace the flat download list with a Downloaded browse surface that
reuses the online grids/cards/detail pages, filtered to on-device media,
plus a demoted Transfers tab. Add repository browse commands
(getDownloadedLibraries/Items, disk usage) with offline/hybrid
implementations, a downloadedCatalog service, formatBytes helper, and
per-item/device disk-usage labels on cards and grids. Regenerated
bindings.

Also carries the inseparable UR-052 offline-filter hunks in
offline.rs/hybrid.rs.

TRACES: UR-055 | DR-081, DR-082, DR-083, DR-084; UR-056 | DR-085

src-tauri/src/commands/repository.rs

@@ -195,6 +195,56 @@ pub async fn repository_get_item(
         .map_err(|e| format!("{:?}", e))
 }
 
+/// Downloaded-only browse: libraries that contain downloaded content.
+///
+/// Backs the Downloads "Downloaded" surface. Never merges server results and is
+/// authoritative — an empty list means nothing is downloaded.
+///
+/// TRACES: UR-055 | DR-082
+#[tauri::command]
+#[specta::specta]
+pub async fn repository_get_downloaded_libraries(
+    manager: State<'_, RepositoryManagerWrapper>,
+    handle: String,
+) -> Result<Vec<Library>, String> {
+    let repo = manager.0.get(&handle).ok_or("Repository not found")?;
+    repo.get_downloaded_libraries()
+        .await
+        .map_err(|e| format!("{:?}", e))
+}
+
+/// Downloaded-only browse: items under a container that are on the device.
+///
+/// TRACES: UR-055 | DR-082, DR-083
+#[tauri::command]
+#[specta::specta]
+pub async fn repository_get_downloaded_items(
+    manager: State<'_, RepositoryManagerWrapper>,
+    handle: String,
+    parent_id: String,
+    options: Option<GetItemsOptions>,
+) -> Result<SearchResult, String> {
+    let repo = manager.0.get(&handle).ok_or("Repository not found")?;
+    repo.get_downloaded_items(&parent_id, options)
+        .await
+        .map_err(|e| format!("{:?}", e))
+}
+
+/// On-disk usage of downloaded content (device total, per-item/container bytes).
+///
+/// TRACES: UR-056 | DR-085
+#[tauri::command]
+#[specta::specta]
+pub async fn repository_get_download_disk_usage(
+    manager: State<'_, RepositoryManagerWrapper>,
+    handle: String,
+) -> Result<DownloadDiskUsage, String> {
+    let repo = manager.0.get(&handle).ok_or("Repository not found")?;
+    repo.get_download_disk_usage()
+        .await
+        .map_err(|e| format!("{:?}", e))
+}
+
 /// Query the optional JRay plugin for the actors on screen at time `t`
 /// (seconds) in an item. Returns an empty list when JRay isn't installed or
 /// has no data for the item, so the caller can render nothing without error.