fix(offline): cancellable downloads + stable sidebar progress toast (#694)

* fix(sidebar): keep offline-download toast from squishing in a short window

The toast lives in the sidebar nav flex column; without flex-shrink: 0 the
column compressed it vertically when the main window was small. The label
now also ellipsis-truncates instead of overflowing on a narrow sidebar.

* fix(offline): make offline downloads cancellable down to the Rust transfer

A running offline download could not be stopped — the sidebar X button only
dropped not-yet-started tracks between batches of 8, and the Rust transfer had
no cancellation path at all, so in-flight HTTP streams always ran to completion.

Add an offline_cancel_flags() registry (mirroring sync_cancel_flags for the
device-sync side) plus additive cancel_offline_downloads / clear_offline_cancel
commands. download_track_offline takes an optional download_id, checks the flag
right after acquiring its semaphore slot, and threads it through
finalize_streamed_download / stream_to_file so an in-flight stream aborts at the
next chunk — the partial .part file is cleaned up by the existing error path.

* fix(offline): cancel per-track and clear the sidebar toast immediately

downloadAlbum tags each run with a downloadId, checks for cancellation before
every track instead of once per 8-track batch (which never re-ran for albums of
8 or fewer tracks), and persists tracks that finished before the cancel so they
are not orphaned on disk. cancelDownload / cancelAllDownloads drop every job for
the album and call cancel_offline_downloads so Rust aborts the in-flight
transfers — the toast disappears at once instead of lingering on stuck rows.

Adds offlineJobStore cancellation tests.

* docs(changelog): offline download cancel button + toast sizing fixes

src-tauri/crates/psysonic-syncfs/src/cache/hot.rs

@@ -69,7 +69,7 @@ pub async fn download_track_hot_cache(
 
     // Stream directly to a .part file; rename on success to avoid partial files.
     let part_path = file_path.with_extension(format!("{suffix}.part"));
-    if let Err(e) = stream_to_file(response, &part_path).await {
+    if let Err(e) = stream_to_file(response, &part_path, None).await {
         let _ = tokio::fs::remove_file(&part_path).await;
         return Err(e);
     }