fix(web,flutter): PlayerBar test scoping + system-playlists revalidate

Two bundled fixes:

### Web: PlayerBar test failures
PlayerBar renders both compact (md:hidden) and desktop (hidden md:flex)
blocks; jsdom doesn't apply CSS media queries so both DOM trees are
present in tests. screen.getByRole/getByText found duplicates →
14 test failures.

Added data-testid="player-bar-compact" + "player-bar-desktop"; tests
scope queries via within(getByTestId(...)). Compact is the focus of
#358, so most tests scope there. The "Up next" subgroup explicitly
scopes to desktop since that copy was dropped from compact.

### Flutter: system playlists not loading
cacheFirst was too conservative — when drift had user-created
playlists from sync but no system playlists (For-You / Discover),
fetchAndPopulate never fired (drift wasn't empty). Result: home
tile row showed user playlists but never the system ones.

Added cacheFirst alwaysRefresh option = stale-while-revalidate.
playlistsListProvider opts in: yields cache immediately, then kicks
off REST refresh in background. Drift watch() picks up the new rows
and re-emits.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/playlists/playlists_provider.dart

@@ -61,6 +61,11 @@ final playlistsListProvider =
       return PlaylistsList(owned: owned, public: pub);
     },
     isOnline: () async => (await ref.read(connectivityProvider.future)),
+    // Aggregate list — server may add system playlists (For-You /
+    // Discover) that the per-user delta sync doesn't always emit.
+    // Stale-while-revalidate ensures the home tile row reflects the
+    // current server state on every screen visit.
+    alwaysRefresh: true,
   );
 });