refactor(device-sync): G.74 — extract helpers + legacy template + fetcher + BrowserRow (cluster) (#641)

Four-cut cluster opening the DeviceSync refactor. 1289 → 1186 LOC
(−103).

deviceSyncHelpers — uuid, formatBytes, trackToSyncInfo (with the
albumArtist-fallback-to-artist logic and the optional playlist
context the Rust sync command consumes), plus the SourceTab /
SyncStatus / RemovableDrive / SyncTrackMaybePlaylist types.

deviceSyncLegacyTemplate — sanitizeComponent (matches Rust's
sanitize_path_component) + OldTemplateTrack + applyLegacyTemplate.
Lives apart from the general helpers because it's only used by the
migration-preview flow and pulls in IS_WINDOWS for path separator.

fetchTracksForSource — single async helper that loads the songs for
a DeviceSyncSource (playlist / album / artist). Artist sources fan
out into parallel getAlbum requests (Navidrome handles them
concurrently; the old sequential loop was a ~7 s blocker on
50-album artists).

BrowserRow — small button row component used by the source-picker
list (playlists / albums / artists tabs).

DeviceSync drops the inline definitions plus the direct getAlbum /
getPlaylist / getArtist imports that only fetchTracksForSource
needed. Pure code move — no behaviour change.

src/utils/fetchTracksForSource.ts

@@ -0,0 +1,18 @@
+import { getArtist } from '../api/subsonicArtists';
+import { getAlbum } from '../api/subsonicLibrary';
+import { getPlaylist } from '../api/subsonicPlaylists';
+import type { SubsonicSong } from '../api/subsonicTypes';
+import type { DeviceSyncSource } from '../store/deviceSyncStore';
+
+export async function fetchTracksForSource(source: DeviceSyncSource): Promise<SubsonicSong[]> {
+  if (source.type === 'playlist') { const { songs } = await getPlaylist(source.id); return songs; }
+  if (source.type === 'album')    { const { songs } = await getAlbum(source.id);    return songs; }
+  const { albums } = await getArtist(source.id);
+  // Parallel album fetches — Navidrome handles getAlbum requests in flight
+  // without serialising. Sequential awaits here multiplied a 50-album artist
+  // sync into 50 round-trips (~7 s blocking) before any device write started.
+  const results = await Promise.all(
+    albums.map(a => getAlbum(a.id).then(r => r.songs).catch(() => [] as SubsonicSong[])),
+  );
+  return results.flat();
+}