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/styles/layout/sidebar-offline-download-queue.css

@@ -11,6 +11,18 @@
   font-size: 11px;
   color: var(--accent);
   overflow: hidden;
+  /* The sidebar nav is a flex column — without this the toast gets
+     vertically squished when the window is short. Keep its natural height. */
+  flex-shrink: 0;
+}
+
+/* Label degrades to an ellipsis when the sidebar is narrow instead of
+   overflowing or compressing the row. */
+.sidebar-offline-queue span {
+  min-width: 0;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
 }
 
 .sidebar-offline-queue--collapsed {