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/src/lib.rs

@@ -417,6 +417,8 @@ pub fn run() {
             psysonic_analysis::commands::analysis_enqueue_seed_from_url,
             psysonic_analysis::commands::analysis_prune_pending_to_track_ids,
             psysonic_syncfs::cache::offline::download_track_offline,
+            psysonic_syncfs::cache::offline::cancel_offline_downloads,
+            psysonic_syncfs::cache::offline::clear_offline_cancel,
             psysonic_syncfs::cache::offline::delete_offline_track,
             psysonic_syncfs::cache::offline::get_offline_cache_size,
             psysonic_syncfs::cache::hot::download_track_hot_cache,